Random Speed change for video

AE is not a video editing app. Ram Previews need to be rendered and most systems can't deliver more than a few seconds of HD playback so you have to learn to work around the limitations. You preview at reduced resolution, you skip frames if you have to, you learn how to use ram previews to verify timing and staging or blocking of your shot, and you learn how to make the final decisions on effects by looking carefully at a few critical frames at full resolution with full effects and motion blur. 

Most of the time you can get long enough ram previews to tell what is going on in a shot. After Effects will not playback a 10-minute movie in real-time because AE is not an NLE.

Either it's an issue with hardware acceleration or the video is a mess and needs to be transcoded in an external program. Impossible to know much, since you haven't offered any info on your computer, your settings, the specs of the video and how it was created and so on, but if the clip is recorded game footage, a screencast or even a clip from a mobile phone there's a good chance it uses tricks liek variable framerates, none of which AE can deal with properly, being that its is aimed at professional footage with constant framerates... In any case, you would have to offer more info to make a proper assessment.

Mylenium