Overheating is certainly a possibility.
If you feel confident doing so, open up the casing and take a look inside. Is it clogged up with dust? If so, clean it out (gently!) with a can of compressed air (but don't allow the fan blades to be spun by the air jet - this can cause damage. Hold them still with a thin stick).
While you're in there, check that all the fans are definitely working. You may have fans on the PSU, the CPU and the graphics card (if you have one).