I confirm anything above 71.4°C when using the Intel(R) Core(TM) 2 Quad processor Q9400 is considered overheating:
Hence, based on the tests done so far, I'd suggest the following:
1. Make sure the processor's using the stock heatsink/fan it came with.
2. Test with our recommended thermal paste; see the following URL:
Note: The surface of the processor can be wiped clean with a soft lint-free cloth and isopropyl alcohol.
3. Just to be 100% sure, check out the hints found here:
Hope that helps!
It turns out the stock HSF just doesn't work very well. I asked others and they all said the stock heat sink was the source of the problem. It was fully functional, nothing blocked nor damaged, and the fan was working fine... but it was just too small to do a decent job.
I was using arctic silver 5 which is supposed to be pretty good. I checked everything on that list multiple times. I reseated the CPU three separate times and after that removed the mobo to make sure it was seating properly and it was.
I was able to get the CPU up to 95°C while the rest of the case was cool. I installed new case fans even.
A friend gave me a pretty basic copper heat sink and I installed it. The CPU now runs 30-40°C cooler than with the stock HSF. This is under the exact same conditions and without any other changes.
Thanks for the suggestions anyway. I had tried all of that already.