Ford 5000 RDS E-O-N with Sony CDXT69 6 disc changer?

This is really starting to annoy me and I have no idea what to do about it. My radio/cd player Ford 5000 RDS E-O-N with Sony CDXT69 6 disc changer was professionally fitted into my Escort Finesse (S reg) in 2001. It is an amazing audio device which I love. Over the last 6-9 months it has developed a problem which is getting gradually worse. I can be driving or parked or waiting at traffic lights, with the radio or cd playing and the CODE message appears on the radio display. I understand this should only happen if the battery gets disconnected. This is clearly not the case. Yesterday the CODE message appeared 7 times on the display. I have to key the code in for the radio/CD to work again. Sometimes the code needs to be rekeyed after just a couple of minutes. Sometimes its fine for hours. Any ideas what the problem might be?