The lens in your PS1 is probably going out. It happens a lot with those; basically, if you turn it on its side when you play it, it forces the laser to read out of another part of the lens, which helps in load times.

It's only a temporary solution, though, as that part of the lens will eventually wear out too. Eventually you'll end up SOL, but this can extend the life of your PS1 a bit. It worked on mine.