erm, just to add something here chaps, im not one hundred percent on this, but it seems to have been the case with all the old gta games and my three previous ps2's.
When you pause the game for long periods of time, like 5 mins in total or ten minutes even, while you get a drink or something, the disc gets scoured by the laser.
the ps2 is reading the same spot on the disc over and over, and it burns the lacquer (varnish) off of the top surface.
You end up with a circular scratch on the disc, and after a while, the ps2 refuses to read it.
i have found that gently cleaning the disc with some multi surface cleaner has reduced the groove and allowed the ps2 to read it again, but in most cases, the gta games just wear out in time.
I cant condone it, but if you go to blockbuster and rent the game, you can always swap it for a disc in better condition.
Not that i would ever do such a thing......
In short, i dont leave mine on pause for longer than a few seconds at a time, while i light a cigarette or something, (or service the missus) i prefer to switch it off, cos i know whats gonna happen if i dont.