NB voltage looks a litle low to me too.... crank the IOH to 1.35 or 1.4. That should help you push a little more. Vtt and pci-e are the usual suspects in the system flaking at high clocks.
Don't be afraid to push the VTT on the 920 to +200 - +250. Heat is a major side effect though. On my 920, the system runs like ass over 4.5 ghz unless the vtt is set to +250. Any thing less or more gives me the red-ass.... gotta find the sweet spot.
