All sorts of factors can come into play causing some disc to read better then others, regardless of scratches, and also, no system is immune. It's simply that the RX has better quality caps that have a better chance of lasting longer. This will still depend on how many hours of play the system has been put through. Also, a dying laser, or one that is not calibrated well, may be able to read some areas of a disc at certain speed rotations fine, but not at others, or have issues with certain disc that have a tad bit more wobble then normal due to focus issues.
You can try to re-calibrate the laser first, and if that wont work, then replace it and calibrate the new one that gets installed. If the laser is still failing then the problem may be the caps or some other component. If you have no prior experience with repairs or this sort of thing then send it to one of the people here who does. Proper calibration especially takes a bit of know how and a certain level of persistence, along with having a good set of disc, both music and game wise, perfect for calibrating lasers to get things performing tip top again.