Causes of CD player skipping?

Hello,
I own a Sony CDP-M555ES 400 CD megachanger that has began skipping pretty badly, especially on tracks 1, 2, and sometimes 3. The problem seems worse with CD-R's and CD-RW's. The tracks will begin playing, and then the player will skip to another place on the track, to the beginning of the track, or to another track.
I tried cleaning the laser lens and this did not work.
Any ideas on what could be causing this?
Thanks for your help.