hi i'm capturing some mini dvd tapes via firewire prem 6 to avis on xp machine and then converting to mp4 using tmpg mastering works 5

the avis are fine but when i convert to mp4 many of the tapes are losing their image (but sounds continues) at various points for rest of the tape
sound is out of sync too

i have tried swapping mini dv camcorders, portable hard drives, laptops, audio rate, std or widescreen and even the mp4 conversion sw - still no luck

any ideas please?

thanks in advance