Nice camera.

Got this phone for about year and half ago. Till now everything was fine. Exept that this phone is symbian based platform, the choise of programs is quite limited. But recently appear one problem, the phone starts to restart alone. When i try open any program, the screen freeze, and the phone restarts! Sometimes during call! but dont do this all the time, have days when it works fine. Take to the service, and they said that don't detect any problem. After 2 days it happened again. And now is apparently ok.
Dont know what to do. I like that phone, got realy fantastic camera, but that problem whith restarting is make me mad!!! HAs anyone have the same problem?