Sounds like the DVD reader is on the blink. Could be dust on the laser lens. You can either ask Sony to repair it (iirc) for a month's wages, or open it up and have a look yourself (likely voiding any warranty). Blow some compressed air around the CD drive then see if it works, and if not, you can get some cleaning alcohol (I'm hardcore and used gin. I don't recommend it) and a cotton bud and give the lens a clean. If that doesn't work you're gonna have to get it repaired.

This is assuming of course that it's a hardware fault. Doesn't seem like it though.