Frame drop during flight

For some time I've been experiencing a strange frame drop after some (variable) time the FPS drop from the usual lock 20 to 8-4. The strange thing is that if I change any hardware option in the settings the frame goes again to the usual value. Tried all software changes, WQL drivers, latest drivers, clean Windows with and without SP2, taking the case covers etc. I start to think about hardware problem.However before starting the new board hunting I wonder if someone has any idea?ThanksJos