why does my cd player keep ejecting cds?

I have a cheap brand cd player.. the laser works fine, it reads almost any cd, re-writables as well, but whenever it feels like it it ejects the cd and switches to tuner mode.
What could be the cause?
It usually does this after playing a couple of songs. it even does this on the motorway where the road is straight with no bumps.. sometimes even when the car is parked..
one thing about it is that it is not installed completely horizontally, does that dact that it has been slightly tilted matter?
Thanks a lot