Audio sync

[K401]

I'm fairly new to dvd copying but I always seem to have the same probelm which is the audio sync. This time it's TMPG, I've encoded a couple of films and everything is fine then I did one yesterday and as the film got further on the audio sync got worse. I checked the audio delay and it was 0m/s so I didn't put it through a audio corrector. Has anyone got any idea's.
Thanks alot
K401