I'm using xilisoft as well for ripping movies!!!
I believe it's because of how it's being converted and the size of the file. For cartoons like Justice League no problem, but for a movie I do get the gap in movement and sound.
for Xilisoft don't rip every chapter in the movie file at once, I find ripping smaller parts like a few chapters at a time and it rips fine without this error.
Or it could be how you are converting them. I let WinAvi (.com) do the converting for me, no problems. I got both of the serials from.... (hm, better stop there.

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Let me know if this helps!!!!