Lag spikes from connection, help!

I have Comcast cable and prior to a couple days ago, it has always been smooth @ 36mbps download and 5mbps upload. I have a Netgear WNDR3700 and Motorola 6121 connected to my PC via Ethernet cable. Up until a couple days ago, I've been getting lag spikes where my games would completely freeze for a good 2-3 seconds and resume, as if the entire connection dropped and reconnected. I did a speedtest and I was getting around 4mbps download and 1-2mbps upload with huge fluctuation in speed.

I called Comcast and they said they were seeing a "red" signal error from their end so they sent out a tech to my house. He came over, changed some cables outside because they were corroded and that's about all he did. I tested my connection while he was still there and I was again getting 36mbps download and 5mbps upload so I sent him on his way saying thank you. It was a quick 1-2 test so everything seemed normal until I started to see lag in games again.

I am getting the same lag spikes, albeit it's not as bad as before but it's still pretty annoying. Using speedtests, I am still getting 36mbps download but the graphs show that there are inconsistencies where there would be sudden drops in speed and picks back up still to show 36mbps. These drops are what I assume are the lag spikes. The upload is also suffering now, dropping down to 2-3mbps and it takes the test about 20-30seconds to load the upload portion of the test. Even loading web pages, sometimes it locks up for a few seconds longer than it should, which is normally instantaneous.

I have tried connecting my PC directly to the modem to bypass the router and am still getting the same lag, probably a little LESS but still the lag is there. My router is has always been updated to the latest firmware. Speed is not an issue as I can still download fast, it's just the sudden drops in speed that causes lagginess which is annoying.

Ideas?