The Disk Drive on my Clavinova CVP 50 has ceased reading disks properly and often operates on its own, freqently 'locking' the keyboard. Can anyone offer any possible reason/cure for this? (other than buy a new clavinova!)

Have you tried to contact Yamaha USA?
They can locate a Yamaha dealer near you, and most of the times Yamaha dealers have contact with authorized Yamaha technicians that can make you a visit and fix the problem (I've seen it with my eyes).