My issues like this always come in bursts. I can have an uptime of 25+ days on my Motorola 3347-02 router, and then for about a week I will have to cycle power on it two to three times a day. It's definitely the router that dies -- I have my main machine plugged directly into it and I can't hit the web interface or telnet into it when things go down.
I've been running it with the cover off for months, so it seems unlikely it is overheating. I haven't tried replacing it yet, but that might be the next move.