I have a Q6600 with a big Zalman heatsink on it, OC'd to 3.0GHz. Idles at just under 50 degrees and goes up around 70 degrees under 4 threads of the P95 Small FFT test.
This is all fine and well, and were the numbers I was shooting for. However, (and it seems to happen randomly), the computer will boot and both CoreTemp and CPU-Z will show that the multiplier changed from 9 to 8, and the CPU is running at 2.6GHz.
I'm relatively new to overclocking, so maybe this is a common problem.