P4 abruptly powers off

I have a fairly new PC on which the CPU fan broke.  After some 'help' from a third party the CPU itself lost a pin.  Replaced the P4 and put a new well specified heat sink & CPU fan in.  Added a case fan.

After running CPU intensive applications for a period the machine shuts off abruptly and will not restart for 10 - 30 minutes, then it works fine.  The CPU temperature fluctuates between 40 and 80 but from what I read this is not a problem as the CPU clock speed gradually drops as the temperature rises.  Right ?

CheckIt (admitedly fairly trivial) CPU tests show no fault.  The fan speeds seem to correctly increase with temperature.   Could a CPU fault be the cause ?   Is 80 degrees really too hot ?
Can I trust the temperature guage on the app which came with the mobo ?

Processor specs at http://processorfinder.intel.com/scripts/details.asp?sSpec=SL79L&ProcFam=483&PkgType=ALL&SysBusSpd=ALL&CorSpd=ALL

Mobo details at http://www.asus.com/prog/spec.asp?m=P4P800-E%20Deluxe&langs=01

Thanks
MN
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EugeneGardnerAsked:
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lordlunacyCommented:
80 Degrees C is far too hot. Intel specs say that your CPU is rated for 69.1 C Max, as listed on the link you posted. This would likely be why your computer is shutting down.

Clock speed on a desktop computer doesn't change with temperature, though fan speeds usually do if the motherboard and/or Fan supports it.

Did you mount the heat sink on the CPU yourself? If so, did you remember to use thermal grease or thermal compound on the contact surfaces? Something such as Arctic Silver ( http://www.arcticsilver.com/ ) is necessary to provide thermal conduction between your CPU and your heat sink, otherwise the heat sink doesn't do a whole lot.
This could be why your temperatures fluctuate so much and rise so high.

My CPU only fluctuates about 6-8 degrees at the most between idle and load, and is almost never over 50 degrees C (Case temperature is around 28 C).
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EugeneGardnerAuthor Commented:
Thanks for the post.  Yes I fitted it myself, used Akasa-450 paste, and added a top notch heatsink & fan.  I think I will have to try returning the CPU and getting another, but I can't imagine what could cause it to get so hot.  Is yours a Pentium4 ?
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EugeneGardnerAuthor Commented:
I think the problem is now resolved.  I added a second case fan and reversed the direction of the first case fan.  Previously the case fan was going in the opposite direction to the PSU fan which was immediately above it.  My thinking was that hot air rises, but I now believe that a local air current was going in a circle.
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rindiCommented:
EugeneGardner,

No comment has been added to this question in more than 21 days, so it is now classified as abandoned..
I will leave the following recommendation for this question in the Cleanup topic area:
Accept: lordlunacy

Any objections should be posted here in the next 4 days. After that time, the question will be closed.


Rindi
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