out of sync

Hi

I want to convert a m4v file to mp4.

At first - one day - it all went fine, both m4v and mp4 all good.

But to day when I try to convert the ok m4v-file to mp4 - the movie gets out of sync?

I use H264 and have tried to export to HD720 standard, HDi720 as well to YouTube 720... so strange.

I am wondering, if I accidentally have changed a setting...

Have any had the same problem? Or what are the right settings?

BR

Zuzette