Why do discs freeze/not read?

Sometimes when I start the system the disc won't read. It spins up, then stops, and never shows the disc as present. It also occasionally freezes and does the same thing while playing a game (with some games there is suddenly an "Insert disc to play" type message). I've cleaned the laser lens with a microfiber cloth, and it doesn't seem to be the door sensor, since the disc spins when the cover is closed, it just doesn't read. Sometimes if I open and close the cover or take the disc out and put it back in, it will read again. Sometimes it will work if I turn it off and back on. If I give up on getting the system to work and shut it down, it may work just fine the next time I turn it on. I have no clue what the problem is. Any thoughts?