VTT 1,3V is too much for 600MHz FSB. It seems that you did not get it Every VTT needs separate CPU GTLs and maybe a VRef as well.
This 615@prime were with 1,25V VTT. Of course every CPU and Board (even with same CPU) needs other combinations, but highest should be 1,28V with right GTLs. You could also try higher ones but that makes absolutely nonsense if you want to push FSB harder, e.g. if you use 1,4V for 600, then what would it be for 650? You get me?