I hope you don't mind me jumping in with a suggestion, but I have had this type of problem before with discs and games. The symptons you describe are as Ori indicated a damaged disk.
This is
most likely a deep scratch on the disc, that is preventing a proper read by the system.
I know this is gonna sound crazy

but before you spend any more money, try the following:
Spread a thin layer of toothpaste on the scratch, and using a damp cloth to wipe off the excess. This should remove the toothpaste residue from everywhere but within the scratch.
As I said before, I've had this problem in the past, and done what I suggested and believe it or not it works !!!
Try googling this technique, and you'll see that it is a known way to overcome this type of problem.
Goodluck! Let me know if it works
Cheers