|There are two main possibilities. One is heat, probably an incorrectly installed or loosened CPU fan/heat sink. The second one is a bad memory card. To test for heat without taking it apart, download a program like Speedfan to get direct readings on your internal components. Report back the range of readings you are getting, especially just before it freezes (all readings, but CPU Core readings and CPU fan RPM readings are the most suspect here). To test your RAM, you have two possibilities here, the first is if you have more than one memory card. if you have two or more memory cards, remove all but one and run your system like that to see if it freezes. Repeat for each card. The second way is better, you download a program called Memtest and burn the ISO image to a CDr. Making sure your system is set to boot from your CD drive first, put in the CD and restart. It should boot to Memtest OUTSIDE your operating system and you should run all of the tests until complete or until it finds an error. If you find an error, shut down and repeat with one memory card at a time to find the problem card.|
If you are getting high temperatures in your CPU and your CPU fan is reporting reasonable fan RPM in Speedfan, you will need to remove your heatsink, clean off the surfaces and reseat with a premium heat sink compound as per instructions.
Report back your findings for more evaluations and advise.
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If all else fails, read instructions.