Audio/video out of sync

When I burn certain .avi files with nero vision 4, the audio plays normally, while the video plays very slowly. I can tell if this problem is going to happen before I burn because in the preview, the same thing happens. So, I don’t think its a burning problem, but an encoding problem. Now, some .avi’s do this and others don’t. I can’t pinpoint the problem at all and its incredibly frustrating. I’ve tried changing the video qualities, audio formats, and encoding mode but I get the same problem. Please help. Thanks in advance.

Also, its weird because if I skip to different points in the movie, it will start in sync but in about 1 second, the video slows again.