Xpantastic!

Chips tend to come up ad loosen during the years. Push all those chips firmly back to their 
places. If you know how to handle the chips you could also remove them one at a time and 
then set firmly back to the position. If this does not help anything, let all condensators be 
replaced and the synth tested by an experienced technician. There are either bad contacts or 
broken chips or too old condensators.