Yamaha MX88 issues with apparent contact switch issues.
Has there been note issues when playing cords.?
Example: when I play simple F cord which is F/A/C at any level a b note also plays at every octave corrupting the sound.
All I see online for something similar to this is the ribbon connector has dirty contacts.
Follow the service manual or look at youtube for videos on how to disassemble your keyboard for cleaning.