It's probably a software design limitation (or bug) in the underlying Illustrator code and there are no workarounds for that. You could dutifully report it here, but that's as good as it gets and even when you do, you shouldn't be expecting it to be fixed any time soon. In 12 years of using AE, this kind of stuff never worked 100% right...
Thanks Mylenium. It looks like the underlying problem (cannot allocate enough memory to antialias file (50:5)) has been around for quite some time. I've tried all manner of different combinations of settings and in the end settled on having it render one frame at a time with dumping cache every 2 frames (hey, at least it renders without running out of ram and erroring out). It's just disheartening to know that due to this issue (bug or architecture) AE is only using a bit more than 1/8 of my machine's CPU power. Perhaps AE should have a feature that has it dump all the memory and attempt multi-frame output again every once in a while during make movie.
I've experienced a similar problem, and have tried to do the same as you. I agree that a reinitializing of the multi-frame rendering every # of frames would be a great way to solve caching issues.