I switched to ATI now, but did you try reverting to an older driver from before this happened? I found that usually it can't hurt, with some of my older NVidia cards the drivers from about a year after release worked best while newer ones could lead to more bugs. I may have upgraded sometimes but it usually didn't seem like a pressing issue and the latest optimizations were usually for games I wasn't particularly interested in, so... YMMV, but I'd try reverting to an older driver.