Vintage Synth Repair

Hi All

I have a problem with a Cheetah MS6 snyth. Some of the tones have
missing or weak notes randomly placed on the keyboard. When I
reselect the same tone the missing/weak notes move position on the
keyboard. Not all the tones do this only a small number but it is 
always the same tones.

Any help would be great as I love this synth and its the only
polyphonic snyth I've got!

Thanks

David.M