In my personal experience Vtt needs to be higher than Vcc but only by 0.02v or so. If you raise Vtt the best thing to do is drop CPU GTLREF to around 0.62 and go from there. Raise Vtt lower GTL, drop VTT raise GTL. Sometimes you need to do both, But 470 FSB on X48 doesnt need anywhere near that Vnb on PL8. Which Divider are you using? Helps if you give that because not all dividers are stable at high multiplier. I have same problem low multi no issues, raise it and breaks most of the dividers. You have same sticks as I do, try use 12:10 divider I find its best personally if you arent already. Half Multis might not be all that stable at higher FSB too. You need to take that into account if possible. Try a full multiplier and see if same problem occurs.

With Vnb I use 1.49v LLC enabled PL7 on 12:10 divider at 481mhz. 0.63x NB GTL.