Choppy playback

[VikingS6]

I'm using MicroDVD to play a .avi movie I just ripped/encoded - What Women Want. I have a Matrox G450 Millennium DualHead graphics card.
When my DualHead DVDMax is enabled, the movie is displayed perfectly through the DVD Max "settings". But with NO SUBTITLES, even though I've made two subtitle files and made an mdvd.ini file wich describes these.

When I disable my DualHead DVDMax, the subtitles are showing.
But then the movie is very choppy and are not running smoothly.
What can be the problem??

Anything to do with the graphic cards driver?? Or the VIA driver??

I also take use of the DualHead Clone feature, so I can really see what's going down on my TV set (a brand new Grundig 29" 100 Hz").

My PC is:
Abit KT7Raid mobo
900 MHz T-bird CPU
768 MB ram

Hope to see some answers here, cause I'm going crazy over this.