Update the Firmware if available . You can use Nokia Suite for checking availability and updating if available ...
If you already have the latest Firmware, re-install the same and check if the problem remains.
Else, try a Soft Reset by dialing *#7370# followed by 12345 (Default code unless changed by you) when prompted. Be warned that this will erase all data from the phone Memory, so back-up important data before doing it. This also can be done using the Nokia Suite. After the reset, again use Nokia Suite to restore data on the phone. You need to re-install the applications.
If still no change, you need to visit Nokia Care