Actually, it is very possible it is conflicting with their game engine somehow. Today I did another clean install and did NOT install PhysX. Made sure I set all the nVidia graphic settings in the control panel to "performance" over graphics as usual, and restarted the game.
What a difference. Screen freezes were not evident even though I went into a raid for a hard mode. Nice and smooth. No problems at all now.
...sigh...certainly would be nice if there was a standard that could be followed easily to make sure game experiences are clean.