There are lots of possibilities. Are you running DHCP? If yes, your lease time could be expiring. Have you set up reasonable security? Changed the default password? ( or set one at all)? Are you using encryption? Remember, these things broadcast, that means anyone with in range, with no security can connect and much with it.
Can you use the hard wired ports after it "carps out"? If yes, go in and look at the status. That may give you a clew.
Can you ping the router when it fails? Post back what you can find out, and any configuration information that deals with the wireless connections.