Can anyone advise me as to why a Region 1 DVD disk won't play without jumping -
on DVD players which have been "made" multiregion?

I have a DVD import which seems to play for about 15 - 20 minutes then starts
jumping.

This happens on 2 different DVD players so it can't be the DVD.

When the DVD is placed in another DVD - or the same scene played at a later
date it doesn't jump. So it can't be the disk?