I went with an eight core through all these versions and I never have had any problems in production.
with rendering.
Just a simple guess: Sounds like a QuickTime problem to me, but well—that is normally a support question.
You might get in contact with them or at least check out if you have QuickTime Pro7 running on your system.
Guessing is not my taste nor really pro like, as you get R13—you might have soon no concerns about R10 anymore,
especially not when you see how much faster R13 is.
Enjoy the 12 core, it is certainly a fun machine.