Huge CPU temperature jumps (not heatsink related)

This is driving me crazy. System is an AMD 4800 X2 on a Gigabyte AMD690 board + 4850 GFX running vista64.

The CPU temp climbs for no reason up to 70+ degrees. Idling can vary from 35 to 70 degrees. Nothing running (no bg apps, no nothing.).

Load the CPU with something like Pi, temp shoots to 80 deg. I can through a bucket of ice in there, it won't make a difference. I can remove the fan from the sink, still no difference. I can add another 10 fans + airconed air, still 80+ deg.

Now the (more) weird stuff: the heatsink is barely warm. Even more weird, as soon as I close Pi and CPU load goes to 0, CPU temp falls -in less than 10 seconds- back to 40 deg.

I would think this is a mobo/cpu temp sensor issue. BUT! The same system one week ago with a different Vista install (same drivers + programs) ran fine.

Even more weird? The PC that this system is replacing HAD EXACTLY THE SAME PROBLEM! Different hardware with XP-32.

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More about huge temperature jumps heatsink related
  1. I need to mention that when idle the CPU temp jumps randomly around 10 degrees. 32-37-29-44-38-48-31-35-49 etc.

    Also noticing that the CPU usage cannot stay flat. I mean it jumps around too. This is pure idling (no sidebar, no email, no indexing, no antivirus, no nothing. What keeps the CPU jumpy? A bit of everything and nothing much. Whereas the System Idle Process should stick to about 90%, it jumps from 50% to 85% then back to 63% and so on.

  2. What stepping cpu? I know that some of the X2s had faulty sensors...
  3. " the heatsink is barely warm" - Hot CPU/Cold heatsink usually points to a problem with the heatsink seating, have you tried removing the heatsink, cleaning off the old thermal compound and reseating with new?

  4. That's what I thought first. Did all steps you mention, same. Then I realised it has nothing to do with the CPU actually heating up, only the sensor going mad (driving with them the PWM fan + thermal protection i.e. shutdown).

    Like I said, I can stop the cpu fan from spinning and there's no difference. I can put a bucket of ice and blow cold air straight to the heatsink, again, no difference.

    The CPU is a Brisbane core. However the system I am replacing with this box had a Toledo X2 CPU and it had the exact same issue.

    Now after trying everything possible I get to this conclusion: It's the addition of the 4850 gfx card that changes things. I don't know how, but with the onboard Gfx I can run 2 instances of Pi and the temp behaves (stays bellow 60 and goes up and down in a normal way). When I plug in the 4850 it all goes bezerk, temps shoot to 80 degrees and go up and down when idle.

    The case is fully open with a huge fan blowing air conditioned air in both setups.
  5. I knew it was the PWM (and all the related hardware/software)! As soon as I replaced the fan on the heatsink with a 3-wire part, temps are steady and low. The climb as they should and come down similarly.

    It's not better cooling (I hold the fan still for testing), it's something with how the system deals with two PWM fans (one on the GFX card, on on the CPU).
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