This is often caused by variable bitrate encoded mp3 audio in your original avi or mpeg4 files.
The Quickstream Fix tool in VideoReDo TV Suite should be able to solve this problem. If not, there is a separate video/audio synchronization tool in VideoReDo as well. This is not a free program, but there is a fully functional trial available for download.
And you might try a different conversion program. FAVC and AVStoDVD are very similar to DVDFlick in function.