OK, so you upped VTT lot, and is slightly more stable, which means that GTL tweaks must be the way for you. Adjusting VTT changes GTL window, but I think becuase you have NB volts so high, alot of GTL tweak is nessecary, and hard to do when you have both NB and cpu GTL to worry about.

Your chip IS NOT qx9650, and my QX9650 works with lower cpu volts than listed above, so pls don't make that comparison. Xello's QX9650 isn't an exceptional sample or anything, it's run of the mill.

Personally, I think you should try default VTT and play with GTLs...find your max stable @ default gtl, and then go a few MHZ above and adjust GTL to suit. I mean, if things were great for you, we wouldn't be having this conversation, so maybe a new approach is a good idea?