I recently purchased a linksys WRT610N router and I have noticed that if I am downloading something for awhile, or playing an online game for awhile, the internet just gets slower and slower. I usually will then run a speed test and I am connected much slower than I should be, and when this happens I have to go to the router set-up page and reboot it for it to be connected at the correct speeds. When it is running the speed test I can see the download speeds stuttering like its trying to go faster but something is stopping it from going faster. I have already been in contact with linksys about this and have done an RMA on it. The second router I received does it also.
I have also had this issue. My only solution thus far has been to reboot the router also. I ended up purchasing a cheap light timer and plugged my router in through it and set it to shut off at something like 4:45am and back on at 5am.
I believe it is a RAM issue that may or may not eventually be corrected in firmware, mostly just lack of RAM in the router though which is a pain to fix. Much easier to purchase a different router which I finally did. Only thing is... the memory issue seems to go unaddressed with linksys. I had to purchase an older used linksys router.
I bought a v2.0 WRT54GS which is compatible with DD-WRT router firmware. I loaded that into it. It comes stock with a 32Mb flash chip and 16Mb of RAM. I've also set a watchdog timer in DDWRT to automatically reboot the router at 5am. I have had zero issues since I did this.