I'm guessing (hence my questions above) that the router is pinging his computer to check it's still up and running, and that his software firewall (Panda) is blocking it, at that point the router will probably stop communicating with the PC. When the diagnostic checker is run the first thing it does is try to establish contact with the router, thus telling the router that the PC is up and running, and thereby restoring the connection.....maybe

---------- Post added at 11:29 ---------- Previous post was at 11:28 ----------
Forgot to ask, are you running the Windows firewall AND Panda?