This sort of symptom is often caused by component (CPU and/or memory) overheating. Make sure the system is clean of dust, and that all the fans are working properly.
rubberman
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What is the make+model of your computer? We need to see if you can get a hardware diagnostic program for it. A lot of manufacturers, such as HP, Dell, etc. have them that you can download and run from CD, floppy, USB thumb drive, etc.
rubberman
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