Your graphics card memory is clogging up. no doubt a configuration issue in teh driver. Also consider not concurrently using acceleration features, i.e. turn off OpenGL in AE to avoid such issues.
Have you installed the After Effects CS5 (10.0.1) update? That update fixes a lot of problems, including some with the "R6025 pure virtual function call" error.
Regarding that error: Every case that I know of involves a conflict with a poorly behaved driver or codec. Make sure that all third-party software (e.g., from Cineform, Black Magic, Matrox, NVIDIA, ATI) is up to date. Several of those companies released updates near the same time that we did to address known issues that casued that error.
Thanks for your suggestions. I've updated all of my 3rd party software drivers, as well as AECS5 and it seemed to work... for an extra 10 minutes. I ended up turning off OpenGL (as directed above) and that has seemed to help as well (although render playback seems to suffer). I will test for another few hours and hopefully it will continue to work.
I was also able to replicate the problem on my laptop with the same proceedures... perhaps it's the ATI card that's causing the issue... but it doesn't seem to be a one-time event. (Although it's causing my laptop to crash now mid-render).
The problem still continues (with errors) even after updating and disabling OpenGL.
I'm going to purchase an Nvidia card tomorrow to see if that fixes it...
It's getting annoying now.