I actually replaced my old router which was a D-Link DIR-628, because it was getting the same problem. I replaced it with a DIR-655 thinking this would fix the problem, however it did not. Usually what happens is it will be flooded with error messages saying that packets are being blocked, then the router will restart itself. Here is the full log. The SSID is hidden, and we're using WPA/WPA2
[INFO] Thu Jan 14 22:36:27 2010 Blocked incoming TCP packet from 65.54.189.83:1863 to 71.192.21.129:50079 as PSH:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:26 2010 Blocked incoming TCP packet from 198.63.196.49:80 to 71.192.21.129:50128 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:26 2010 Blocked incoming TCP packet from 198.63.196.57:80 to 71.192.21.129:50134 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:25 2010 Blocked incoming TCP packet from 198.63.196.49:80 to 71.192.21.129:50128 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:25 2010 Blocked incoming TCP packet from 198.63.196.57:80 to 71.192.21.129:50134 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:24 2010 Blocked incoming TCP packet from 198.63.196.49:80 to 71.192.21.129:50128 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:24 2010 Blocked incoming TCP packet from 198.63.196.57:80 to 71.192.21.129:50134 as FIN:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:23 2010 Blocked incoming TCP packet from 65.54.189.83:1863 to 71.192.21.129:50079 as PSH:ACK received but there is no active connection
[INFO] Thu Jan 14 22:36:21 2010 Above message repeated 1 times
[INFO] Thu Jan 14 22:36:19 2010 Blocked incoming TCP connection request from 74.214.184.142:54374 to 71.192.21.129:26550
[INFO] Thu Jan 14 22:36:19 2010 Blocked incoming TCP packet from 65.54.189.83:1863 to 71.192.21.129:50079 as PSH:ACK received but there is no active connections
Quite frankly it's really annoying, and I'm not sure what's worse, the fact that it's still broken after buying a new router, or that buying a new router didn't fix it.