Which distro? It might be a chipset issue, it may just be the program you are using, what do you mean by: "calculation program"? Does the machine freeze up under heavy load every time.
Check through /var/log/messages for a line that includes: "Machine Check Exception" and some memory address... that typically means the machine is just overheating, look in BIOS and see what the core is idling around.
Cheers,
Finegan
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