Sounds like it could be a touch pad. The rubber fatigues and cracks which causes the carbon knob of that particular note to cock to one side and make that note unresponsive.
They are about $23 from Yamaha,Ca..Part number VU328400 (a 12 note rubber section)
1(888)892-6242 If you are mechanically talented, you can replace this part. If not,
you need a techician to do it. It requires removing the bottom of the keyboard.Larry C.