Can you give us more information? What kind of connection do you have and who is your provider?
What times of the day is it resetting?? What are you exactly doing when the disconnects are happening (playing online games or using bit torrent)??
Have you changed the default password to your router? Did you scan your computer for viruses??
If your router is "rebooting", there are only a few things I can see possibly happening.
1. Hardware is faulty
2. Configuration issue
3. Someone is somehow gaining access to your router (trojan on PC) or (access to web interface)
4. Power supply issue
If your router is "disconnecting" and you have already updated the firmware, replaced network cable etc..
1. You may be reaching your maximum concurrent connections
2. Your ISP may be experiencing network issues or performing maintenance.
Leave the router unplugged for atleast ten minutes and plug it back in. Don't use any file-sharing programs [p2p, bit-torrent, etc..] See how long your connection stays up.
Report back.