GTA3 for Xbox: Unreadable

I have a legal copy of GTA 3 for Xbox that somehow became unreadable recently. It seems that it has only a few minor scratches to the polycarbonate layer, the kind that should only generate a few PI errors, and none to the foil layer. However, the XBOX freezes up after a few minutes, while loading a scene, or tells me that it is unable to recognize the disc. Except for the first 13 megs of it, I am unable to test it in my DVD burner on my PC for PO failures that would cause such errors. What are some possible reasons besides major scratches to either the foil or polycarbonate layer why a disc would become unusable? Also, is there any software for testing PO/PI errors of XBox games in a PC?

Have you tried other games? Maybe the drive is on its way out.

Yes, everything else works fine.