Just got my PS2 out from the cupboard, and noticed it has gone a little weird:

It plays DVDs just fine with no issues at all, and plays about 2 in 10 of my PS2 games perfectly and loads first time. But... And now here's where it gets weird, it WONT play any PS1 games or 8/10 PS2 games.

Weird or what? Any ideas what this could be?