I don't really understand the question. What is a "pre-animated clip?" Pre-rendered, maybe?
And what do you mean by "frames that won't animate properly"?
If you're talking about Prerenders, possibly you're rendering to a non-editable codec such as H.264?
I refer you to Dave LaRonde
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.
Also, I've seen Render Multiple Frames Simultaneously cause problems on some systems, so if that's on try turning it off.
Finally, disable OpenGL if it's on. I think Dave might have a stock answer for that as well.