I know it sounds simple but....
I wanted to slap myself
Ok, so I thoroughly read all the posts on the other forum that you linked to and became increasingly depressed as my build and problems were identical. Some games ran super smooth while other choppy and basically unplayable with my enthusiast OCD.
My build:
Win 7 x64 ult
Asus sabertooth p67
16 gb corsair vengeance ddr3 1600
Core i7 2600k
Asus engtx 580 dcuii
The previous poster on the forum was right about the power source being the issue, but incorrect on how to solve it, at least in my case. I stumbled across this:
Make sure windows is configured to performance and not balanced or efficient.
Go to:
Control panel -> power settings and check the performance bubble (if it isn't showing when you get there look down and click to show hidden profiles and it should be there)
I'm assuming this why my old 9800gtx+ could run smooth with the two 6pin and the 580 couldnt with two 8 pins, windows was limiting the power consumption.
Please let me know if this solves your issues.