DVD Player Hesitating and Skipping

I own a Dimension E520 desktop with Vista. I use it a lot to watch DVDs in my bedroom, but lately there's been a couple issues. For the first twenty or so minutes of playtime, everything is fine. However, after that, my computer begins making a strange thumping sound. Following that the DVD I'm watching skips or hesitates - the video freezes, but sometimes I can still hear the audio playing. After a few seconds the DVD video speeds up, apparently trying to "catch up." This happens repeatedly in each sitting, often continuously. I typically have no other programs running while I watch DVDs and everything else works fine memory-wise. This doesn't just happen with one particular DVD, so I know it's not the disk.

Note: I'm new to the forums, so I may have posted in the wrong area. Sorry!