GGAAHH! I have the same problem with my Disc Four. I dropped my FFVIII box from quite a height one time whilst I was sorting out my CD drawer. Thus, I suspect scratching and the disc transition to Disc 4 has never been the same since...

The only thing I can think of is that the discs have somehow gotten dirty or damaged. Invest in some disc cleaning solution, or some repair shops/ game shops may offer a disc repairing service. Look in the Yellow Pages or similar for such a service in your locality.

One other thing, check you have the right game in. Once whilst I was playing FFVIII, I tried to change from Disc 1 to Disc 2. I must have tried a dozen times to get the disc to work, and I was absolutely sure it was disc 2. Finally I looked at the disc one more and found it was Disc 2 from Final Fantasy IX!

So ummm... yeah...