I suggest you download and try one of these two programs which I have listed below, to get a basic idea of what your CPU core temperatures are, this will give you an idea if your problem is heat related or not.
Speed Fan http://www.almico.com/sfdownload.php
Depending on your CPU's threshold temperature, if you see a temperature higher than 80 degrees Celsius, this would be the culprit.
Please check your CPU cooling fan for dust and check that it spins freely when it is spun with your finger, it should spin several times without binding up. If it binds up, it is in need of replacement.
Make sure there is not any dust embedded in the fan and the cooling fins of the heat sink which is attached to the CPU.
If you see dust build up, you need to remove this as it is acting as an insulator and causing the heat to remain stuck in the heat sink.
Purchasing a can of compressed air, will help remove embedded dust.
While you're at it, blow out the rest of the computer as well.
Hope this helps.
Please post back the results.
Kind regards as always.