sync problem - have suffered for weeks, please help

I just can't get this to work, please help! When I encode my AVIs to mpeg (VCD) the sound perfectly matches the video at the beginning. However, as the video gets longer I always get an increasing difference between video and sound. The sound is always faster then the video. I use TMPGEnc to encode. No, I don't use the computer while I encode. The problem does not occur when TMPGEnc recognizes the sound itself, but often it doesn't and then I create a sound file (WAV file) with Virtual DUB or Graph Edit before I start the encoding process and use the created wav file as an audio source. The odd thing is that my WAV file is just as long as my AVI video. I'd really appreciate all help I can get. Thanx in advance,
Joe