wildly varying cpu temperature

I did a big rebuild on PC No3 last week with a new mobo (foxconn socket 754) and sempron 3000. The foxconn has onboard LAN sound and geforce 6.
Its crashing every ten mins or so, and when I installed Everest to take a look I see that the CPU temp is reporting different every second or so, showing numbers like 8, 30, 16, 60, 40, 12 etc like that. Clearly this report is not true. I'm assuming this must be the m/b sensor and the crash is caused by the occasional 80+ reading? Are there any tests I can do or other explanations, before I take it back and look like a fool when there's a simple explanation? :(

Bad power
Failing Fan

Assuming that's not the problem, I'd take it back, too. But I believe the sensor for temp is actually in the PROC, not the mobo.

I wouldn't necessarily assume that the readings are untrue - if there isn't good thermal contact between the CPU and heatsink then temps can rise amazingly fast. Have a look in the BIOS and see what it is reporting thge temps as in there.

I agree with Sparks, make sure the heatsink is firmly attached to the cpu, use a good thermal paste like Arctic Silver5 after you clean the old stuff off the cpu and heatsink. If you decide to return the board see if you could get an Asus, Abit, Epox, Gigabyte, MSI main board. Better boards make for more stable systems.

The heatsink is definately neatly seated with a thin smear of arctic paste. The temp goes rapidly DOWN as well as up so I don't really think this can be the issue. Is ProfessorU right that the sensor is on the CPU and not the mobo?

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showing numbers like 8, 30, 16, 60, 40, 12 etc like that.

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With temp readings like 8 you would have to be in the Antartic with the side cover off and a stiff north wind. Your bios is not showing the correct temps at all. Its not unusual for cpu temps to very depending on load and room temp. Also many mainboards have onboard cpu temp controls that will very the fan speed according to cpu temp. My 32bit system has a sensor right under the cpu, I am not sure about my 64bit system I will have to dig out the book to see.

I don't think for a minute they're correct, I'm just looking for the reason they missreport. Actually it seems to be just Everest - in the bios the temp shows as steady 45 degrees which I think is OK so I'll have to look elsewhere for problems

Everest might be reporting a different sensor that is not the proc temp. I haven't used it lately but try to reconfigure it to identify the processor temperature correctly by selecting the appropriate sensor.

With the new mobo Everest reports a nice steady CPU temp which looks right - 42 degrees and no crashes