Possible causes -
1) It disagrees with your taste in music
2) Something has entered the slot and is obscuring the lens, making the player think that it is seeing no disc, so spitting it out.
3) The sensor (if there is one) which tells the player that there is a disc inside, is faulty, intermittently.
Remedies -
1) Throw away your Barry Manilow CDs
2) See if you can get the hoover nozzle in there, or a can of air with a thin tube which can get in the slot.
3) Take it out and shake it about. (The player !)
4) All Three (just to be sure)
Good Luck
Chris.