Help! Very unique cable modem problem

I am experiencing connectivity issues with a cable modem: every hour it slows down to a crawl for a few seconds and can't figure out what causes it. This causes my IP phone to drop calls every hour.

Here's my config:
PC with static IP address connected directly in to a Motorola Surfboard SBG900 using an cat5e cable. The SBG900 also is configured with a static IP (tried with DHCP and it behaves the same way).


[1345136311.416352] 64 bytes from icmp_req=392 ttl=64 time=2.23 ms
[1345136312.416436] 64 bytes from icmp_req=393 ttl=64 time=1.01 ms
[1345136321.712487] 64 bytes from icmp_req=394 ttl=64 time=8295 ms
[1345136321.712678] 64 bytes from icmp_req=395 ttl=64 time=7287 ms
[1345136321.712698] 64 bytes from icmp_req=396 ttl=64 time=6279 ms
[1345136321.712884] 64 bytes from icmp_req=397 ttl=64 time=5271 ms
[1345136321.712904] 64 bytes from icmp_req=398 ttl=64 time=4263 ms
[1345136321.713090] 64 bytes from icmp_req=399 ttl=64 time=3256 ms
[1345136321.713110] 64 bytes from icmp_req=400 ttl=64 time=2248 ms
[1345136321.713323] 64 bytes from icmp_req=401 ttl=64 time=1240 ms
[1345136321.713343] 64 bytes from icmp_req=402 ttl=64 time=232 ms
[1345136322.483181] 64 bytes from icmp_req=403 ttl=64 time=1.01 ms
[1345136323.484275] 64 bytes from icmp_req=404 ttl=64 time=1.02 ms
[1345136331.787879] 64 bytes from icmp_req=405 ttl=64 time=7303 ms
[1345136331.788065] 64 bytes from icmp_req=406 ttl=64 time=6295 ms
[1345136331.788085] 64 bytes from icmp_req=407 ttl=64 time=5287 ms
[1345136331.788270] 64 bytes from icmp_req=408 ttl=64 time=4279 ms
[1345136331.788290] 64 bytes from icmp_req=409 ttl=64 time=3271 ms
[1345136331.788476] 64 bytes from icmp_req=410 ttl=64 time=2263 ms
[1345136331.788496] 64 bytes from icmp_req=411 ttl=64 time=1255 ms
[1345136331.788690] 64 bytes from icmp_req=412 ttl=64 time=247 ms
[1345136332.543546] 64 bytes from icmp_req=413 ttl=64 time=1.03 ms
[1345136333.544637] 64 bytes from icmp_req=414 ttl=64 time=1.01 ms
[1345136334.546495] 64 bytes from icmp_req=415 ttl=64 time=1.78 ms
[1345136335.547591] 64 bytes from icmp_req=416 ttl=64 time=1.02 ms
[1345136336.547883] 64 bytes from icmp_req=417 ttl=64 time=1.00 ms
[1345136337.548971] 64 bytes from icmp_req=418 ttl=64 time=1.01 ms

As you can tell, the ping is pretty fast (~1ms) most of th etime, but when the top of the hour comes around, I get 2 periods of decreasing delays (8, 7, 6 ... seconds and again 8, 7, 6... seconds).
This repetitive pattern suggests a recurring external factor, but can't figure out what it is. Perhaps the cable company is injecting packets periodically to determine the status of the link? One way or another, I'd like to resolve this as it drops calls and remote terminal connections.

Anyone knows how to further troubleshoot this? Whether it is a modem or a network issue? Has other inputs that can help resolve this?
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More about help unique cable modem problem
  1. Seems like the kind of thing you need to take up w/ the ISP, esp. if they provided the equipment. Because once you're connected directly to the modem over wire, and you're doing nothing more than pinging the modem and observing this predictable and repeatable behavior, there's not much more you can do (perhaps resetting the modem??). Something is probably up between your location and the ISP that only they can address.
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