In the past 3 months i have been getting constant lag spikes in everything and they happen like clockwork about every 8 seconds in all games. I can mostly tell in games like tf2 because i see my ping go from 20ms and spike to 300 ms for 1 second then go slowly back down to 20-30ms every 8 seconds. I am not on any wireless connection, this happens with or without my router. When I phoned my isp they said that everything was alright on their end and that my connection is stable. I have tried disabling my firewall, anti virus, switching cables. This problem is basically rendering any online fps unplayable.
Running windows 7 64 bit
onboard atheros ethernet
onboard HD audio
gtx 260 216
asus p5q pro
d-link dir 615 router
I guess my first thought would be to try another computer and see if you still get the same issue. If you do then you know the issue is not being caused by your system. At that point you call your ISP back up and tell them that you have the same issue across multiple machines.
At this point(unless you own your own cable modem) it is in your ISP's court. ISPs are very notorious for passing the blame onto your computer just because it's a lot easier than trying to do the work to fix the issue themselves.