There isn’t ant difference between the final Cd you copied & one that has been made from MPR or similar, both Cd's you end up with will have the .wav format used by Cd players. Have you started using different media lately? The only other thing is copying MP3's to a audio disk does place a greater load on your Pc as the Pc is converting the files from MP3 to .wav while burning.
Perhaps you could try writing at a slower speed & nothing running in the background as the Pc could be turning out less that brilliant disks. Some older Cd’s by nature don't like burned Cd's & this coyld be tipping these players over the edge.