same issue with 6 core, windows 7 64-bit with 12GB ram and nvidia GTX 460. made one change that made playback smooth.
nividia control panel > 3d setting > manage 3d settings > adobe premiere selected from list > muti-display/mixed-GPR acceleration > changed to single display performance mode.
this worked perfectly even though i have two monitors and i just assumed the single display performance mode would not work for me.
good luck