OK, first off the modem and router are separate pieces or hardware. Comcast gave us the modem and we bought the wireless router. Signal strength is full and windows reports 54mbps in my room a floor above the router even when speeds are slow.
This happens to all wireless computers, my family has done weekly virus/spyware scans, so i don't think there is a virus.
I don't believe we are being hijacked, i know passwords can be broken but the one we have is pretty long. I don't think the wireless signal would drop to slow every 20 minutes if that was the case.
I did some debugging:
when i reset both modem and router at the same time: about 20 min of fast speed.
when i reset just the modem the router stayed at the slow speeds.
even at only 3 feet from the router, the speeds are still slow.
when the speed is slow on wireless i can hardwire in and get full speeds.
I'm no expert on hardware, im a software guy, but it sounds very much like its a bad router.