If you haven't already, I'd start with installing latest updates to OS X and make sure you have the latest firmware. Also repair permissions with Disk Utility. If problem persists, make sure no peripherals are attached. If problem still persists, create a new account in OS X and boot into it. If the problem disappears that way then it's a software issue. Some login item you can find in the account preference pane. You could also try and boot into safe mode. If none of these tips helps, it's most likely a hardware issue.
Joshua.
