Is it all footage or just a certain type?
If that's not it, you may be previewing with OpenGL turned on (turn it to adaptive resolution instead).
If that's not it, it may be the type of footage you're using. If it's HDV, Dave LaRonde from CreativeCOW.net explains the problem:
If the footage you imported into AE is any kind of the following -- footage in an HDV acquisition codec, MPEG1, MPEG2, mp4, m2t, H.261 or H.264 -- you need to convert it to a different codec.
These kinds of footage use temporal, or interframe compression. They have keyframes at regular intervals, containing complete frame information. However, the frames in between do NOT have complete information. Interframe codecs toss out duplicated information.
In order to maintain peak rendering efficiency, AE needs complete information for each and every frame. But because these kinds of footage contain only partial information, AE freaks out, resulting in a wide variety of problems.
Hope that helps.
Thanks, we tried turning Open GL off and it fixes the problem, which is weird because it was working just fine the other day. At least we can work the through the probelm, but I think it is just a band aid.
The footage is either in Avid codec or JPEG.
It's odd that it worked in the past, but isn't working now.
Have you updated your OS or video card lately? Have you tried trashing your AE preferences? Any chance previewing with OpenGL was off before, but just got turned on recently?
No, the only thing we did was add 4 gigs of RAM about a week ago. I will try to trash prefs and start over.