How to fix the lagging audio of a converted AVI file?

Usually to save hard disk space I convert downloaded files from say a 640x400 resolution MPG, AVI or WMV to a 320x240 standard DIVX encoded AVI file and usually the conversion takes place without a hitch but sometimes the audio of some files just lag in a progressive manner, to say it's starts ok but loses sync as the video progresses so that by the time the video is finished the video and the audio are one mile away from each other! So what is the procedure to fix this. I do have VirtualDubMod and VirtualDub as well, so I have the tools but would like to know how to fix this particular type of audio lag using the programs I mentioned before. I know they are used for this stuff but not the correct way to use them to fix this so any help would be greatly appreciated Thank you! I've trird Skewing the audiot track dubbing and replacing it with Demux and nothing seems to do the trick! The Audio starts in perfect sync with the video but as it is been playback the audio slowly but surely lags behind so that by the time the video ends well as I said in relative terms to a race the video is at the finish line and thr audio is a quarter of a mile behind. As I said I've tried just about everything to no avail so if anybody knows what is the right and better way to fix this plz tell me so! I'm desperate over here!