I have a Phenom II x4 965 BE and an ASUS M4A79XTD EVO mobo. When overclocking I can get my bus speed up to about 220 stable, but putting the multiplier so that core speed is greater than about 3.7Ghz always makes it unstable, no matter what voltage. Even lowering bus speed and increasing multiplier isn't stable. I have a few theories, but would like others opinions!
Heres what I think:
A) PSU can't provide enough power for the CPU
B) Got very unlucky with chip and that's the limit. (I thought PII's could average about 4GHz?!)