Looking into my crystal ball i see i need the following info to help you better!
When did the problem first start?
Had you installed anything new or a new program?
Whats your Computer specs ie; O/S, RAM etc.
That's usually, but not always, a result of overheating.
As TXD mentioned, give the innards a good clean out and check all fans are spinning.
You could also try taking the side of the case off and directing a domestic desk fan inside and see if this alleviates the problem. If it does - overheating is the problem.