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