Its more than likely that the white gear inside is misaligned. If not under warranty (im sure your not) and if you feel comfortable opening your console, reply and I will post pictures on how to fix.
First off, before you go and adjust the white gear, try to put some tape on the label side of the purple disc by the middle without covering the hole. I will post pics if needed.
If the tape doesn't work that the laser probably needs to be adjusted. Follow the steps on this link and tell me if it fixes the problem. http://www.tweak3d.net/joomla/tutorials/hardware-repair/playstation-2-repair
these "purple" disks you speak of are CDs. This could be the laser needing to be aligned like core said, but this happened to my ps2. I was using swap magic, then my ps2 just wore out. So realign now before its too laaaaaaaaaate