Scott, you’re experience with motion graphics dwarfs mine. I don’t know whether these will help, but there are a couple of things I’d try.
Since you’re on a Mac, does the Activity Monitor provide any clues to memory usage? Does AE crash? If so, does the crash reporter provide any clues?
Have you tried 1. Setting the Secret prefs to purge every x number of frames, 2. the old standby of resetting AE preferences.