Yeah, I don't know why, but the PS2 sucks sometimes trying to read PS1 disks. I have the same trouble you did - every single time I try to play the game.

Ergo, here's how I jerry-rigged it into working (under the assumption it's not playing Disc 1):

1. Put in another disc and wait for it to load. (Try it with all of the discs until you get ONE of them to work.)

2. Start your game and select the "continue" or "new game" option if you're starting fresh.

3. When you select "new game" or your Disc 1 file, it will ask for you to insert Disc 1. Open the CD drawer and insert Disc 1. Now the PS2 will more or less have to read it! Yeah! Error this disc, beeyotch!

I talk to inanimate objects too much.