.m4v videos play jerky with VISTA

sound is OK, but video is jerky. These have been purchased from iTunes and they worked fine with Windows XP. I've tried all the troubleshooting suggestions on Apple support site. Anyone else have a solution?


Take a look here and see if any of the tips help:
Troubleshooting iTunes for Windows Vista video playback performance issues
The suggestions for Vista are somewhat different than those for XP.
Good luck.