Vintage Synth Repair

Dear All
 
 Any body can give me a help. My Minimoog had some problem on the
 keyboard. Actually it got some problem only on the particular keys.
 When I press the problems key it can not stay at a pitch. It will
 floating . Just like to use the LFO to control the Osc Freq. It only
 had 4 keys had this problem. But when I use midi to control the
 minimoog(yes my minimoog had install midi). It had no problems. Any
 body can ask me Why? How can i repair it?
 
 Bird