Could try cleaning the laser. There are cleaner discs.
I prefer to dismantle drive and clean lens with a cotton wool stick. Hold the stick at the far end to exert minimum pessure on lens focus mechanism use a gentle alcohol or propanol solvent, dry off with dry cotton wool stick.
Lite-on service suggested cleaning the laser and also resetting the unit to the factory defaults. Doing both those things seems to have solved my problem. Thanks