When I start AE (CS6) 10.0.2 (Win7, x64), previewing a 10-15 second scene with multiprocessing works fine and is very fast. After making 3-7 different RAM previews by pressing numpad 0, AE will no longer generate these previews, and instead will either say that it's preparing to do so, or stop at having rendered "2 frames out of x". I can see that it has stopped working because in the Task Manager, the different instances of AE will not be consuming more RAM to prepare for frame renders. AE and my system will still work fine, and I can still view the RAM previews that I made beforehand, but to get MP to work flawlessly again, I need to restart the program.
I'm using an 8 core Xeon processor @ 2.6 ghz, with hyperthreading enabled, and with 32 GB of RAM for this processor. I've read that decreasing the amount of available cores can help with AE multiprocessing. I've tried that, as well as playing with the amount of RAM that each core gets, and it still will inevitably hang. I've also read that problems like this can be due to hard drive bottlenecks. When I look at Windows Resource Monitor, this isn't a problem at all. I'm using AE to make compositions with JPEG time lapse sequences that can be anywhere from 2,000x2,000 px to 25,000x20,000 px large, and often with multiple JPEG time lapses together. It's for this project in case you're curious.
I spent a lot of money on this system, and plan to add another 8 core Xeon processor along with another 32 GB of RAM just to get faster renders (it's a dual socket ASUS board). Hopefully there's a fix for this flakey multiprocessing situation, or will be an update in the future. I think turning MP off does work, but it seems needless to do something 8 times as slow, or to have to always restart the program just to use a basic feature.:
EDIT: I should also say that rendering final compositions with multiprocessing ususally works. For 2-5 minute projects, it almost always works. Rendering 30-70 minute compositions sometimes works. Sometimes it crashes during these longer ones, and I have to try repeatedly, and eventually it'll work. Sometimes doing things like emptying the Media Cache works.