New Mondeo Cd problem?

I have a Sony Front loading 6 cd changer in my new mondeo it recently got a new unit fitted as it was jumping and not playing certain CDs. Then it eventually would not play at all and said CD Drive Fault.
Now my new one is just as bad and i have bought brand new cds as they told me it could be my cd's. This has not solved anything. It won't play them at all and then just ejects them and says please check cd.
Somtimes it will play them then it does'nt. Somtimes they will keep jumping through the first three or four tracks then it does'nt play and sometimes it will play can you sugest anything as Ford are starting to think it's me. They checked this unit and said there was no fault found. They are all genuine cds.