Quick tip – you can delete afx’s preferences by holding Ctrl-Alt-Shift (Windows) or Command-Option-Shift (Mac OS) as you start after effects, no need to dig into your system preferences.
Found this on Creative Cow. I guess it works better than dragging the prefs file to the trash. It must reset more properties than dragging the prefs to the trash.
Part of the update was 4GB of RAM to bring me up to 12 and a new video card. ATI Radeon HD 5870 Graphics Upgrade Kit for Mac. Not sure if the video card had something to do with it.
It’s odd that it occurs randomly like that. From what I’ve seen on Creative Cow and Adobe’s forum, the solution you found is what should correct the problem.
Maybe the last resort is to uninstall and reinstall AE?
Like you said, I’m not sure if there are any issues with the new video card you installed. How much RAM do you have allocated to AE in the preferences? Maybe try increasing that, or enable disk cache?