previously same problem.
Same MoBo, same cpu, same heatsink.
That board has cpu overheat protection - it will shut down.
I found that after checking the bios and fan connections and resetting the heatsink several times that the heatsink was really not seated.
The step that goes over the cpu clamp was not wide enough and was hanging up slightly on the clamp. It looked seated but was raised a bit on the one edge causing the cpu to overheat. If it were not for the protection built into the board the cpu would have smoked. Thank you Asus !
Anyway, I happen to have access to a milling machine and was able to widen the step on the heatsink. I probably took off about 1/8". You could positively see that the edge of the step was between the clamp and the cpu pad. Now the heatsink was definitly sitting flat on the core. This took care of the problem.
If you think this could be your problem but do not have access to a machine, a file or Dremel would work fine. It does not need to be perfect. Just make sure you don't remove too much copper so the sink does not sit completely on the cpu pads.
And, if you got the new Coolermaster Heatpipe with the 'silent' fan - ditch the fan. That 14 cfm or whatever it is just doesn't cut it. You will see temp creep into the upper 50s to mid 60s. Get a fan with a little more push.
With a decent fan my heatpipe is keeping the XP2100 at 38c idle and tops out at 45c after running Sandra for 30 minutes - thats with 30c ambient. I am happy with that.
For it is not what is seen, but what is not seen.