Did you try the obvious -- raise CPU voltage? I need to set 1.425 in BIOS to get stable at 4.33GHz. It gives me 1.408V actual voltage in Windows (read by CPU-Z) under load. Idle is 1.392V. That's OK with me, as I'd rather not exceed 1.4V for 24/7 use.

What's your CPU voltage currently set to?