Every time I try to preview what I am creating in AE it will start to render, but when it gets to a certain length it will unrender what it rendered at the beginning. If that makes any sense???
I can still make a movie, it only happens when previewing.
It's a RAM preview, so you're limited by the amount of (physical) RAM you have. Do as chez says and lower the quality of the preview, or just go out and get more memory.