This new performance option is bullcrap , it does not fix the clocks jumping and down like a yo yo while using windows 7. Not sure what it's like in vista .
Im staying with what works 186.39!
It was caused by aero! just hovering over mininmize/maximise/close buttons will cause it to jump up, as do other aero features. setting basic theme solves it, even with the sidebars present.
So aero IS causing these jumps with the new nvidia drivers