the codecs might have break,
or rather the burn process was interrupted, or
bad dvd have been used......
have you try burning other title to other brand of dvd media??
i suggest u to get 1 or 2 DVD-RW or DVD+RW so that you can use the re-writable media to check the result....
What software are you using for the burner? Nero, Sonic, Record Now, CD Creator, Power DVD?
It may be time for a new optical drive. Very low prices these days.