Scribblings from the Mitchy Side

I'm having major problems playing DVDs on my computer, but only film dvds. Playback is fine at first but after about five minutes the playback becomes choppy and rapidly degenerates until it is unwatchable. I've updated every single driver I can think of, I'm using the latest version of DirectX, I've got the latest K-lite codec pack installed and the DVDs appear to be in good condition, no major scratches or flaws. I do not have any problems with other types of video playback - mp4 and avi files are just fine. And, weirdly, I haven't had any problems watching DVDs of TV shows, just films. The problem exists across all the media players I have on my PC.

I'm running Windows XP which has been regularly updated. I'm currently triple checking for viruses and unwanted spyware but I have done this before and nothing improved. Graphics card is NVIDIA GeForce G11, sound card is RealTek Audio.

Any ideas guys? I've about reached the point where I'm thinking it's gotta be the DVD drive being freaky and I'll just buy another one. :P

ETA: doccy has walked me through a few things, like deleting a spare Primary IDE Channel that seemed to be redundant, tweaking the Registry, making sure DMA was being used, deleting some recent windows updates to see if they caused the problem etc. So far no joy. I have been able to play some DVDs successfully but not all. I can't find the common denominator. *sighs*