modem? or cable problem? (jagged peaks in speed tests...)

Thanks Boosted -
I found the log but I don't see how to attach a screenshot, though I have one at the ready.

Log has one T4 and two T3s (excerpted below) from the 15 total entries. What do it mean?

Mon Jul 20 17:01:31 2015 Critical (3) Received Response to Broadcast Maintenance Request, But no Unicast Maintenance opportunities received - T4 time out;CM-MAC=00:8e:f2:30:72:b8;CMTS-MAC=00:01:5c:6b:66:6e;CM-QOS=1.1;CM-VER=3.0;

Mon Jul 20 03:21:40 2015 Critical (3) Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=00:8e:f2:30:72:b8;CMTS-MAC=00:01:5c:6b:66:6e;CM-QOS=1.1;CM-VER=3.0;

Time Not Established Critical (3) No Ranging Response received - T3 time-out;CM-MAC=00:8e:f2:30:72:b8;CMTS-MAC=00:01:5c:6b:66:6e;CM-QOS=1.0;CM-VER=3.0;

Thanks again Boosted - but when you say "...80% likely problem is between house and network hub...", the hub to which you refer is TWC's hardware, eg, out on a pole somewhere - IOW "not my problem?"

Followup:
Cable guy A came on Wednesday and spent a lot of time checking the connections, everything from the pole-mounted junction to the back of my modem, testing the line in segments as he went. He changed all fittings, but in conclusion, said his 'meter' (something like an LCD oscilloscope that I don't know anything about) was still showing a lot of noise problems in the line, so he would coordinate replacement of the line to the pole for another day. He didn't have a test-modem with him, or didn't want to make yet another trip down to his van to get it, which was understandable since he was convinced that it was a line problem anyway. When he left, my speed-tests were in the neighborhood of 50MBPS, which is way better than it has been recently, so I was OK with it, at least short-term. Still, he was certain there was still a problem and said I should be getting about 300MBPS.

Cable guy B called a couple of days later, saying he was "A's" supervisor, and that he wanted to look at things for himself. He showed up with a loaner modem and installed it. Right away, my wired connection through the loaner showed a huge speed improvement, reading over 300MBPS, and I was ready to toss my modem into a tree-chipper. But it took well over an hour for him to work through the setup of the loaner, as he was having all kinds of problems getting wifi to work. When he left after a couple of hours here, it was working - but it crapped out about 2 hours later, when I could get no connection whatsoever. Since there wasn't any operating software (as I'm accustomed to using for the Netgear) I had only the Windows control-panel selections to diagnose connection problems, and those were not coming up with anything, though I'm way less than proficient at navigating those items.

I called Cable Guy B, though it was after hours for him by then, and he suggested I reconnect my old modem until he could come back. I did so. And guess what: the old modem now is giving me steady, non-jaggy speeds, consistently upwards of 100MBPS. OK, it's not the 300MBPS they say I should be getting, but it's clearly a vast change/improvement.

So...WTF? Seems to me like all of these bits of data contradict each other and there's still no clear diagnosis.

Is there a Netgear setting somewhere that allows the max speed to be throttled/unthrottled?

Thanks Boosted - Cable guys ''B" and "C" were just here, and seemed as clueless as I as to why these changes in speed have been occurring. And really, I didn't know for sure what speed I SHOULD have been getting until yesterday...ill-informed. They were ready to install and leave a third (huge) loaner, but since I'm at least for now OK with the speed on my old modem and poised to purchase a new one, I let them leave without any further fuss.

One looked up my existing modem and concluded it was only good for 100MBPS, but looking over his shoulder, it looked to me like there were some different letters appending/prepending the basic ID of CG3000D. I'd googled the other day and not found anything conclusive about this modem's capabilities - just ads from retailers claiming either 200 or 300MBPS, either of which would be fine for my purposes. Got anything on that?

Regardless, since it seems flaky, my plan now is to buy the modem they sanction (per their suggestion) from Costco, an SBG183, Arris/Motorola. I'm accustomed to having some "management" software for the Netgear, which has been my only (user-friendly) window into modem/wifi status and diagnosis. Is there something similar for the Moto? (Or is there a better choice of modems to begin with?)

boosted,

I keep thinking this is resolved, but more questions arise. I've had rock-steady 118.18Mbps for a week since factory-resetting the modem, which I'm actually kind of OK with, but TWC service people keep reminding me I'm paying for 300, so...

Regarding your "4 vs 8 bonded channels" - the Amazon seller of this modem indicates that he is quoting the original Netgear blurb for the modem with this (excerpt): "...Features: Video Features 8 Downstream plus 4 Upstream Channel bonding on the WAN, delivering; Up to 400 Mbps..." (it goes on with other specs and has the appearance of being taken from some typical Netgear specs, but they're not available from Netgear - see below.) Since I don't really speak this language, I'm wondering if the specs or that claim is at odds with your previous conclusion regarding the 4 bonded channels. (Does the word "video" belong there?)

I'd thought this should be as simple as going to Netgear to look it up, or if need be to ask, but they've got no published documentation, and when I dug into their support link, it looks to me like they don't want to touch these modems with a 3M pole...even though they were their own product...apparently since Netgear sold them en masse to Charter Cable, who for some reason dumped them such that they ended up on the Amazon/ebay/web-retail market, where I bought mine.

Amazon seller also suggested that TWC would have a firmware upgrade, but I think anyone who tries to look up the CG3000D ends up with little or nothing that's actually directly useful - since Netgear is doing everything they can to orphan it? Is the firmware upgrade a reasonable proposition? (so far TWC has come back to me with the same conclusion you did regarding 100Mbps limitation, but they cited a slightly different model number, so...)

Boosted - thanks for the quick & informative reply. I could have found this information where you suggested before, if I'd looked thoroughly. Here's pasted info from the router page:
--------------------------------------------------
Startup Procedure
Procedure Status Comment
Acquire Downstream Channel 567000000 Hz Locked
Connectivity State OK Operational
Boot State OK Operational
Configuration File OK
Security Enabled BPI+
Downstream Bonded Channels
Lock Status Modulation Channel ID Symbol rate Frequency Power SNR Docsis/EuroDocsis locked
Locked QAM256 1 5360537 sym/sec 567000000 Hz 8.2 dBmV 41.4 dBmV Docsis
Locked QAM256 2 5360537 sym/sec 573000000 Hz 8.2 dBmV 41.5 dBmV Docsis
Locked QAM256 3 5360537 sym/sec 579000000 Hz 8.5 dBmV 39.9 dBmV Docsis
Locked QAM256 4 5360537 sym/sec 585000000 Hz 8.3 dBmV 41.4 dBmV Docsis
Locked QAM256 5 5360537 sym/sec 591000000 Hz 8.1 dBmV 41.4 dBmV Docsis
Locked QAM256 7 5360537 sym/sec 603000000 Hz 7.4 dBmV 41.5 dBmV Docsis
Locked QAM256 9 5360537 sym/sec 615000000 Hz 7.5 dBmV 41.2 dBmV Docsis
Locked QAM256 10 5360537 sym/sec 621000000 Hz 7.7 dBmV 41.4 dBmV Docsis
Upstream Bonded Channels
Lock Status Modulation Channel ID Symbol Rate Frequency Power
Locked ATDMA 36 5120 Ksym/sec 35800000 Hz 43.0 dBmV
Locked ATDMA 33 5120 Ksym/sec 14275000 Hz 43.0 dBmV
Locked ATDMA 34 5120 Ksym/sec 20975000 Hz 43.0 dBmV
Locked ATDMA 35 5120 Ksym/sec 29100000 Hz 43.0 dBmV
--------------------------------------------------

I'm not working now (hence lots of time to pursue this...my live-cheap factor has definitely kicked in) so I'll either keep trying to tweak this modem to get the speed up, or live with it until I've got some income, then return to your recommendations for purchase. I think I paid about $60 for it over a year ago, so even if it were to be at its limit now, I'd still feel OK about having to live-with or replace it.

Maybe it's worth going back to TWC again to convince them there should be a firmware upgrade after all? The TWC guys I've been dealing with here are really extremely helpful; where I'd have expected them to give me only the sanctioned corporate response of recommending that I rent a modem, they're instead actually telling me which one to buy from Costco!

Boosted1g - got an interpretation of my pasted router-page specs? That is, are we talking 100Mbps limit, or 400, or something else entirely?