Yamaha Clavinova CLP-155 odd symptom

Here’s a stumper…

The E, F, A#, and B keys in all octaves only work on the first stroke.

After that, they are silent, for a while. After a random period of time, the affected keys work again…once.

If power is cycled, the same symptoms reappear.

Removed the rubber key hammer contractor strip and cleaned. Also cleaned all contacts for the three circuit boards that the contractor strip touches. No change in symptoms.

Used an oscilloscope to watch key depressions, the o-scope registers keystrokes for all keys, even when the stroke produces no output from the piano.

No obvious circuit board faults. No leaking or bulged capacitors.

Any ideas?