I have the same problem.
I have been advised that it is either a dirty or faulty laser.
Looking into both scenarios, you need to open the machine up and clean the laser with an alcohol dosed cotton bud. Using a cleaning disc will not work as the machine will not read the laser cleaning discs either.
Should the cleaning not work a new laser assy will set you back £45 - £50 and you have to fit it (available on e-Bay). Cheaper to buy a second machine off e-Bay !!!!!!!!!!!!
Rob Farey, January 2009