Every time I've had a problem, I have traced it to the router, not the phone. I live a long way down the copper line, and my ADSL was getting interrupted so I had my ISO slow the bit rate and increase the reliability. Now only drops out during thunderstorms and blackouts.
@ Meiga, have you tried connecting to a reliable public (unsecured) wifi? If you can, it sounds like a security issue. Just a suggestion.