LipSync

Ive been downloading Lost since the start of Season 2 and everytime I watch it on my PC the lip sync goes out. Its ok to begin with but gradually goes right out of sync. If I close the media player and re-open it and then forward to the point I was up to its ok again.....strange. Ive tried Nero, WMP, DivX Player and BlazeHDTV player but it happens on all of em.

Also when I convert and burn the .avi to DVD or SVCD the same thing happens...it starts off ok but gradually goes out of sync.
I watched one the .avi files on my work PC and it was fine all the way through so I can only conclude that it is my PC causing it to go out of sync.

Does anyone have an ideas why it might be happening or what I can do to fix it ?

Thanks