Hmm, almost sounds like for some reason the nvidia drivers and AA in KDE are not getting along. I don't recall having seen anyone else report anything similar. Maybve something just with your hardware configuration.
Might check the forums on Nvidia's website and see if anyone else has posted the same problem.
Only other thing I can thing of is it doesn't like some of the fonts you installed when AA is enabled. But that seems a little strange to me.