I purchased a Kurzweil K2600 once and the very upper key would not work. I reinstalled the firmware and that fixed it. I am not saying that is your issue, however, if all else fails you might want to give it a try. It could also be a bad solder joint on the socket of the keyboard. Have you plugged the sustain pedal in and tried wiggling the cable where it plugs into the keyboard, while you depress the sustain pedal a few times. If it works intermittently it could be a bad solder joint.