Hi,
I have had the same problem with my unit. I tried all sorts of things and in the end fixed it by ejecting all of the CD's and then filling the unit with 6 CD's. It has worked fine since then. According to a friend the internal heads can become missaligned and by filling the unit it brings everything back together.
WARNING: make sure that you can eject discs before you try this as it can be very costly to get your discs back if they get 'locked' inside the machine. My advice would be to use 6 non-original discs i.e. ones that you could effectively afford to loose.
Andrew
September 2008