Have you overclocked any component of your computer? If you have, try default clock settings.
How long have you had these issues? Since the beginning? Or when you installed a component?
It could be a couple of things:
RAM malfunctioning, do a memory testhttp://windows.microsoft.com/en-us/windows7/diagnosing-memory-problems-on-your-computer
Graphics card: Either overclocked, overheating, possible bad driver, or malfunctioning card
PSU: Last option of malfunction.