Hard reset router, set it up again, check for unknown background processes running, lose the Zonealarm or Norton bloatware (buy a good firewall instead) and report back.
When does it reset itself? During master server list? During gameplay? On crowded servers? On empty servers? Playing single player? Eating nachos while watching replays?
Usually some cheap routers (Zyxel, D-Link, etc.) go haywire and reset themselves if they receive too many connections.