PC restarted or died when computing

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http://askubuntu.com – I just updated ubuntu to 12.04 (precise) 64-bit. When I run some code in matlab, the computer is always dead or restarts. I couldn't figure out the reason. Although video card on the mother board is only 32mb, I have nvidia vedio card and choose primary video card to be other in BIOS. Can you give me some suggestion? Maybe first, I can start with some test to see health of computer. Thanks in advance! leo (HowTos)