WK-3200 Repair Help with single key playing multiple notes at once

My Casio keyboard plays eight notes at once on some keys even shows this on the display, starting at the low A, then every eight notes F, C#, A, F, C#, A, F C# all play the eight notes around then at the same time when I play just the one note. I have cleaned the contact boards and the rubber contacts, and checked all solder connections. All other functions and sounds are perfect. Anyone know how to fix this