Hi Mark,
Are you sure that your mains lead is firmly seated in the back of your keyboard and that the plug in the wall socket is correctly fitted. If the mains lead in the back of the keyboard is loose, then slight movement of the keyboard, when pressing the panel buttons, could cause the connection to become intermittent.
Since I don't know which part of the world you are located, I don't know if you have a fuse fitted inside the mains plug. In UK, the mains plugs we use, have a cartridge fuse inside the plug body and I have known them to make an intermittent connection, on more than one occasion..........
This type of fault could produce the effect which you describe.
_________________________
Willum
After silence, that which comes nearest to expressing the inexpressible is Music.
Aldous Huxley
( especially when the music is played on a KN7000....)