I had a similar problem recently. When on line my connection (56K modem) would drop after about two hours. When I attempted to redial the connection would fail. Only way to get on line again was to re boot. After analysing my system in this state, I discovered that a third party had tried to hijacked my system and was trying to dial a premium number. Because this type of call is barred by my phone provider there was no obvious signs other than a small icon on my task bar. I deleted all the evidence but the problem just kept coming back. It was only when I installed some spyware protection software that the problem finaly disapeared. Perhaps your problem is a similar one?