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Why
It could be several things but the most likely cause is insufficient cooling of the CPU.
Check if this is the case by downloading something like PC Wizard and see what temperature your CPU gets to under load.
http://download.cnet.com/PC-Wizard-2010/3000-2094_4-10793737.html
If the CPU is actually overheating and exceeds 80 deg under heavy load then check the fans are working properly and check that the vents are not clogged with dust. You can try using a can of compressed air carefully but often the laptop will need taking apart to clean it properly.