DVD Skipping

I made a DVD of YouTube videos using Windows Live Movie Maker. The amount of video on each disk was about one hour and twenty minutes. When that skipped, I tried making another disk of the videos--this time a half-hour shorter--but I have the same problem. It never skips in the beginning--always in the middle and getting worse as the disk continues to play.
The file type is .wmv and the DVD is a Memorex DVD-R disk with 4.7 GB (about 120 minutes) worth of space.
What can I do to make a DVD that doesn't skip? Should I try a different format?
Thank you!