If the ax1200 upgrade doesn't fix it, then that's my next step, I'll probably get the x58a-oc just to make sure that there's no way in hell it can be a vrm limit. I don't need any more features than it has anyway. If it's still a problem then I guess the chip is degrading.
I've seen plenty of power supplies advertised as 500w and they were only outputting like 375w.
It very much could be a case of not enough power, especially when your kill-a-watt says you're pulling 650w/ 550w at PSU.
Get something that will handle what you have and want in the near future like say a solid 850w and you should be golden.