Don't set the voltage over 2.3 volts. If it don't fly at 2.3 it won't work at 2.5 either. And yes, your temp is stopping you, but only because you're already at the limit of core speed. If you cool it a little better it will stabilize at 605MHz. Of course it might have to be cooled to room temp or less to do it. You have found the limit of air cooling.
P4 email@example.com dual R134A phase change chillers with 600watt thermoelectric array. Capable of -70F
Nixie tube numeric display control panel and pneumatic motherboard tray.