Your hardware is up to scratch so it sould not be that (although getting the lasest Detonator drivers from Nvidea's site might be a good idea), one posibility is to see what is running in the background when your'e playing the DVD's.
Sometimes the computer gets a bit bogged down doing other things and messes up the video, when this happens it wait's for the next "key frame" to come along and re-synchs the video with the audio (FYI for your pc, the audio takes practically no processing time to decode, but the video does).
If your PC is just too busy doing other things then it will miss even the key frames and the video will seem to hang untill it can find the next and it has enough time to decode and play it)
Try re-booting your pc, exit any non essential programs that start automatically and try again.
Hope this helps:)