This could be a bunch of things. Your comp frame rate could be different than your footage and you may have Preserve Frame Rate of nested comps turned on. You could have .....
Well to be honest, without knowing what version of AE, What OS, or anything about your composition or your footage there's not much I can say.
As Rick said, we need to know a lot more - footage used, footage interpretation, comp settings, system specs, what version of Ae, is audio used and so on and so on. A common cause for the behavior you describe is audio problems where AE tries to retain the audio speed, but the video whacks out. Anyway, at this point it's a "I could ask my mom" thing. We simply don't know enough.