Could be dust or dirt/oil on the laser or receiver inside the player. Lasers gradually diminish in output, reducing the player's ability to correctly receive the data from the disc. Another possibility is the transport (mechanics) itself causing the skipping. A cleaning might solve the problem; best done by a tech as you do not want to accidentally misalign the transport. There could also be a problem in the decoding electronics but that is less likely IME.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley