I totaly agree with Wayne's diagnostic: Over heating!
Your heat sinks are probably clogged with dust, lot of it!
That dust acts like a thermal isolent and prevent the air flow from reatching the heat sink.
Place yourself outside.
You will dislodge a large amount of dust, and been outside can prevent some ulterior cleaning of your house.
Open the case.
Locate the heat sink(s) and gently blow some DRY and CLEAN compressed air between the heat sink's fins to dislodge the dust.
Start with the CPU heat sink. Easily located and accessed, just under that fan on the motherboard.
If you have some high performance video card fitted with a fan, do the same for it's heat sink.
Electro, April 2011