I suggest that you to run your phone in safe mode for a while to check if you still experience this. If not this may be caused by any downloaded application.
How to boot into safe mode?
If you still get this even in safe mode I suggest that you try to reinstall the phone software using Update Service..
Before reinstalling your device you may want to backup your information first. Check out this topic for more information on how to.
How to backup?
If the issue should still remain I think that this needs to be examined and fixed at a repair center. For more information about how to submit your phone for repair and where, contact your local support team.
