Problem with hp DVD 840e

I recently upgraded my PC, and to my annoyance, I still cannot play DVDs properly. The problem seems to be that I can’t enable DMA on my external DVD drive. I have updated the firmware, but I still can’t enable it. I have tried many DVD players, such as PowerDVD and VLC, but it still will not play properly.

The DVD plays fine for the most part, but for any length of time between a few seconds and five minutes the sound will go all jittery and sound awful.

Can anyone help me out? (I’m on Win 2K SP4).