I think you use the wrong CPU GTL's. If i was using the wrong GTL's i would have problems to get it stable around FSB 400MHz.

I tried Mikeyakame's suggestion to use CPU GTL 0.65X and NB GTL 0.63X and higher VTT with my Q9450 on my RF board. This makes it completely unstable. Perhaps if i would use multiplier 6 instead of 8 it might be stable.