What version of After Effects? What operating system?
If it's CS5, have you installed the After Effects CS5 (10.0.1) update? That update fixes a lot of problems.
Does this happen with every composition or just this one?
Finally, try disabling the audio layers and try again.
It's CS5 on Windows Vista, but it's only the 30-day trial version.
As for the audio, I've tried disabling it as well as removing it altogether, but no change.
Try installing the update that I linked to. It fixed some RAM preview problems.
I downloaded the update, but there was no change. I also tried it on a different computer/OS (a Mac at school), but I still got the same result. I'm running out of ideas.
> Does this happen with every composition or just this one?
You didn't answer that question.
Try doing a RAM preview with a very simple composition---such as a composition with just a single solid-color layer.