help please.....problems with cd player in citroen c5 ?

Hi, i wonder if anybody has had this problem, or can help me.... my cd player in my citroen has always been troublesome and has only played intermittently.
I have never had trouble with the radio,just the cd.
It would almost play when it wanted to! We would flick through the songs in the hope that it would start playing and sometimes it would insert itself and play, and other times it wouldnt do a thing,other than ejecting itself.
Now, it will not play at all when the engine is running,but ......works just about 1st time whenever you switch it on, as long as engine is NOT running.
I would be really really grateful if anybody could help me with this before i end up hitting it and breaking it completely!! :o) Debbie