Most of MY problems (or ALL of them) with DVD burning are related either to:
1. Using cheap media
2. Using scratched media (scratches around the edges frequently cause errors in the final phase of burning, since the burning is initiated in the center of a CD / DVD)
3. Opening lots of apps during burning
4. Opening few CPU-demanding apps during burning...
By the way, I also use Toast... Aren't you by any chance leaving the " Prevent Buffer Underrun" checkbox unmarked, are you??? It should be left CHECKED...
As for the Finder vs Toast issue, test it and post it...