Blupunkt Woodstock losing memory ?

My in car unit is the DAB 54. Its about 5 years old. All the supply cables are intact and undamaged. The problem only affects the CD player the radio memory is fine. I can be playing a disc get half way through and then leave the car for either a short time or longer, when I switch on the disc starts again at the first track. It always used to start from where it was left off. I wonder if there is an internal battery which needs replacing or something similar. I tested it by turning the car off and then on and it continued playing the same track, I went into a shop and came out 5 minutes later and it started again at the first track, so it does have a short memory at times. All suggestions are appreciated (Sensible ones ...)