I had an issue with burning when I 1st got this computer. I had to disable some of the authoring extensions. But that was OS9.

What is your burner drive brand & model? Generally Mac use Pioneer for their dvd burners. But not always. I wonder if yours needs a driver update (I can't think of what you call it. Purely guesssing.)

And you tried Toast with buffer under-run protection. Next suggestion is to update your version of Toast. The later versions of Toast 6 give much better buffer underrun support.

I also came across these results: HERE which may lead to an answer! (Click on link)