Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Could also be from overheating or a component that's broken. Monitor your temps with istat or a similair app. Concerning dust use a fan control app to set your fans on the highest rpm to try to blow out dust.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.