Have you tried upping the core voltage? You're safe up to 1.55v. On my previous MB I had to bump up the voltage to overclock my x6 otherwise I had the same problem.
I don't know if this is the best way to do it or not, but its what I did. Bumped to 1.4 and that took me too 3.6GHZ no problem then to 1.45 to 4.0GHZ. That's as far as I pushed it and settled on 3.6 for 24/7.