It could be due to your memory & processor allocations. Here are some useful guidelines to follow:
http://blogs.adobe.com/toddkopriva/2010/10/please-try-recommended-memory-se...
The short version -- don't use even close to all your processors & all your memory for AE, because bad things will happen.
If you haven't run the updates to AE 10 since you installed it, you should probably do so. Updates fix a lot of strange AE behavior.
Finally, you're sort of in the gray area of multiprocessing. 12 gigs may seem like a boatload, but not for a 64-bit application. I have a dual quad core machine with 8 gigs at work, and I don't use multiprocessing. I have a dual quad-core machine with 24 gigs at home, and multiprocessing is fine.
Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
