Well, after completing the factory reset, cleaning all the contacts and plugs and using switch cleaner on the slider switches.....it still has the problem !

I can only assume its a dry solder joint or a loose conection somewhere on the amplifier circuit. I will try to connect the line out to an external amp and see if the sound loss still happens. Hopefully it will narrow down the search for the problem.

If nothing else, it give me the opportunity to play the keyboard every evening as I tell the wife I'm testing it !!!