omnia 7 - apps automatically exit, and freeze the phone

Hello, I am Csaba Hungary, is a type omnia 7 my phone about a month of purchase calculated worked correctly, but then when I tried to 3G Internet, the phone automatically restarts. So I took the service (T-Mobile), software is updated it and gave it back. It also solved the problem, but instead became another problem. The phone automatically exits from any application without notice, including integrated applications (Internet Explorer, music player, Now) if I try to go back into it, then it back a few times, then suddenly stop responding and automatically restart the phone. So again I took the service and came back that they did nothing with it, because during the tests in principle with the phone had no problem ...
What is in their opinion of this?
sorry for my bad english :(
Any anwser?