cd player not reading a disc? Laser dirty?

What causes this erratic situ where sometimes the player won't fire up a disc that it was playing a 1/2 hour before?
Could it be the laser is dirty? I clean the disc(again) to no effect.
Will it play other disks or just not the one that was already in the transport? Also, do you leave your CD player powered up all the time or turn it on for each use?
It is erratic. Sometimes not the one in the disc; then may not play another I have played recently. I may move the disc in the tray and it may play after 1 or 2 attempts; or I may put in another disc and it plays. Only happens with CD's not DVD's.
DVD's always play...?? It is frustrating.
I think its all going to depend on what kind of transport the unit has. I can't see the problem being anything else. It sounds like the laser may be dirty, as you suspect, or possibly something to do with the tray itself. Given the fact that you sometimes get the disc to play after you move it in the tray. I don't know what player you have, but I think it would be a pretty safe bet to try and locate a new transport. If you are not afraid to open the unit up, its probably a part that you can easily replace yourself.
Sometimes there is a capacitor in the power supply that directly feeds the laser diode ... when that capacitor begins to fail it will intermittently not hold/store the charge and keeps the laser from powering up ... eventually it will fail completely and the laser will not power up at all

Just another possibility to check out
Have it serviced or pop the cover and do it yourself. Youtube has beaucoup videos on how to clean the player and lens.
Hey, thanks Mental. Never thought of looking for that on u-toob. My 3910 gets erratic disk read errors and eventually stops reading altogether. I had to bring it in for service, but I bet I can follow a DIY video. I'll give it a try.
You can buy a laser lens cleaning cd disc at best buy for about 10 bucks. Just a series of soft little brushes on the disc. They work pretty good. I'd try that before anything else.
FWIW. The problem is worse when the unit is well warmed up, say after playing a dvd.