sony dvd recorder rdrhxd710 fault?

I have a sony dvd recorder it doesn't wont to record lately and comes up 'disc is dirty' i have tried loads of different blank dvds...
Anyway today i found out that if i put a dvd-r into the sony,then changed it to dvd+rw it starts recording for a short time before playing up again and then i need to swap dvds again,it appears that its not changing over from - to + properly
Any ideas how to sort this out without having to keep changing them over during recording?
Thanks Tracey