Thank you Ted.
If you are not up for it, maybe you can find a friend familiar with electrical circuit to see if they can figure out where the problem goes.
As far as the keyboard reliability goes, it is quite reliable but . . .
The keyboard consists of 2 parts. The etched copper gold plated interwoven "combs" right on the PC board are bulletproof.
The other part is the silicone rubber membrane with those black "pucks" or discs. Those are also rubber, but they are coated with black carbon-rich conductive paint.
When a key is pressed down, the conductive surface of the "puck" creates electrical connections between the "tines" of those pads on the PC Board.
During use the conductive paint can wear down, or debris can get under the pad and degrade the conductivity. The rubber itself can also leach out some liquid which also degraded conductivity.
Over the years I have successful revived many of those keyboards (in remote control or calculators). I have some remote controls which are about 30 years old which I revived multiple times.
The repairs revolve around cleanup or restoring the conductivity to those "pucks". If the carbon paint layer is worn down there are inexpensive carbon-based conductive paints out there, but most will just flake off the rubber surface. The one that works (probably the same stuff used in manufacturing those keyboards) is a special 2-part paint, and it is a bit pricey. Usually available from electronic parts supply houses (I got mine from Digikey in USA). Thankfully NCE offers replacement keyboard membranes. That makes it easy to fix the keyboard if the problem is with the membrane or the "pucks".
For cleaning the metal pads and those "pucks" I use 91 or 99% isopropyl alcohol. In your case it seems that there is something other wrong than just dirty keyboard.
