I own a WRT54GL wireless router with the latest firmware (as of this posting) on a cable internet connection. My PC is using a wired connection, not wireless. I had been noticing that my connections over Remote Desktop from work would experience frequent but short periods of very high latency. I finally got sick of this because it was making RDP unusable for the most part.
I downloaded PingPlotter and had it running on 1 second intervals. I discovered that the *maximum* amount of time between one of these lag spikes was about 1.5 minutes. Most of the time I would get lag spikes separated by just 10-20 seconds.
To prove the problem was the router, I connected my PC directly to the cable modem and ran PingPlotter again. No lag spikes this time. Next, I uploaded the latest Thibor custom firmware to the router to see if it fixed the issue. It didn't. Finally, I disabled most of the router's features including wireless access, the firewall, etc. None of them caused the lag spikes to stop.
I've unplugged my wireless phone (I have Vonage) and that did not fix the problem.
At this point the only option I can see is buying a new router unless someone has dealt with and fixed this problem in the past.