Hello, my name is Rusty. Is there any pattern as to which notes are out, or is it random?
Ok, I know what the likely cause is, are you able to open it up and do the repair yourself? (with my guidance of course...)
You'll need to remove all of the screws from the bottom of the keyboard, then the top console flips up to expose the keys. There are contact strips that are struck when you hit a key. Those strips send a signal to the tone generator that makes sound. When the contact strips fail, it causes intermittent key outages. They are a maintenance item and they are still available.
To get a new strip, call(###) ###-#### ***** through Friday, from 7:30AM - 5:00PM Pacific Time (closed 12PM - 1PM). Let me know if you have any trouble getting it open.
Which model Clavinova do you have? Click here, they are all about the same. Go to 1:04, and you'll see what I'm talking about. When you remove the screws from the bottom, the top flips up to expose the electronics. You'll see the contact strips on the back of the keys, no need to remove the front.
Did you find the contact strip? It's where the keys hit when you strike them.