By "made hundreds of discs from this project," you make no changes to the project, and keep burning the same build? In my opinion,k there are too many variables to count on Encore not changing something. Build to an iso or folder, and burn from that.
The most likely cause is that you inadvertently changed something.
You could try posting a screenshot of your project and build panels.
If I misunderstand, and you are changing the project in any way, that is a different story...