This kept happening to the router that I used to have. It was a Linksys BEFSR1 (I forget the exact model number, but it was the first model of that little purple box thing). It lasted for about five years before it started to just cut out on me.
It was really annoying, too, especially for long downloads. It would be fine for days, and then it would stop. I'd unplug the power, wait, plug it back in, and everything would work for about an hour and then it'd die again. Unplug again and then it would work for another couple of days.
I finally got pissed off enough to just go out and buy a new router.
I never figured out what was causing it; I asked some guys at school who know more about networking than me, and they said that there's no reason why it should be doing that. It's either something that's going on with the external network that makes it crash, or the router itself is dying. It was really dusty, so that could have been the problem. :/