Thanks to GJ for the suggest to use IMGBURN. I downloaded and used IMGBURN to create and then burn an image of one unplayable disk and it now plays just fine on my Sony BRD player!
I had switched from VSO to using Nero as a burning engine a while ago and was pleased with the improvement, but this is perhaps better still. I am unsure the tech differances or pros/cons between these 3 burning engines. But IMGBURN works and it's hard to argue with that.
Thanks again to GJ, and also to Troy512 for taking the time to reply, appreciated!