Hello out there,
I had that issue of freezing Premiere Pro CS6 when i try to switch between the Software/Hardware of the MPE ... it was a bit frustrating ... and i decided to go back to CS5.5 for stability issues ...
But i think i came around a solution to the issue, i think the issue could be in the Dynamic Switching in the MacBook Pro itself (Intel/ATI) ... so i fount this FREE utility that is called gfxCardStatus
http://codykrieger.com/gfxCardStatus it enables you so switch on/off this dynamic switching, so i gave it a try and put the OSX to ONLY use the ATI card, and it works fine for me right now :)
Good Luck,
Amr Toukhy