It works now!
I upgraded to 32 GB RAM today (Crucial - used their system scanner to select the correct RAM for my system.) I also installed the latest EuControl software and hooked up an Avid Artist Mix and Transport. I just tried to open a session now with PT 11 HD and the video engine loaded this time. It took about 10-15 seconds.
I'm assuming the RAM upgrade fixed it?
If you're having the same problem, try installing the maximum amount of RAM your system can handle, using the Crucial system scanner to select the type.