From my brief testing (that is, just trying to watch a damn dvd!), it seemed to be fairly random. Sometimes I could watch the half hour episode, sometimes it would lock in 5 minutes.
I had (at that time) lots of stuff installed - including Vista Codec Package and Cyberlink PowerDVD. I also changed DVD drive, and that made no difference (that wasn't because of the problem, but I just happened to buy a new one - the new one was SATA, ruling out the crummy JMicron PATA controller drivers).
Try updating your graphics drivers too - other than not reinstalling PowerDVD, that's all I can think of that might be different. And I haven't seen the problem since my format... so I'm hoping I've cured it.