Half-way burn?

We have two Sony DVD RW 720A(s). We have had absolutely no problem burning movies for the past 2 years. As of late, we have had multiple problems burning movies. I can't even recall the last time we were successfully able to burn a movie. The CD burner works perfectly fine, but when you attempt to burn a movie, it will say "copy completed" - we use 1 Click Copy - but if you try to watch the movie, toward the end of the disc, it will start getting choppy and freeze. You can actually see a spot on the bottom of the disc where something went wrong.
Sometimes, it will say "copy completed" and the disc won't play at all. And at other times, the disc will act like it burned okay, but there will be less than half of an inch of content on the bottom of the disc, so it definitely isn't right.

Our PC is up to date on everything. We have Windows XP. Everything else is working great... it's just the DVD burning part of it... We're really confused....

Any help would be greatly appreciated!!!!

-Jess & John