The reason for that cavalier answer is that there's no information in it to help us diagnose the problem. So here are some of the more popular solutions to AE weirdness... but because we don't know a single thing about your system, none of them may help:
Upgrade to AE 9.0.2. It's a free download.
Turn off Open GL for rendering. It's mostly useless in AE, and often harmful.
If you only have 4GB RAM or less, don't use multiprocessing.
Don't use compressed audio like mp3 or ac3, Use only aiff or wav files.
Don't use temporally-compressed video. Here's an explanation:
Dave's Stock Answer #1:
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.
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