Vintage Synth Repair

Hi, 

I have a Roland MKS-30 that is acting weird. I am hoping someone 
here can help me with it.

I use a Roland A-33 controller with my MKS-30.
 
The problem is when playing a patch from the MKS-30, certain keys do 
not sound when they are pressed. If I change to a new patch, those 
notes will now sound, but others will not. It is a very random thing 
each time I use it.( only a few notes on the keyboard will not 
sound, but they are always different.)
For instance it is sometimes a "D" and an "F", then I change the 
patch, and it is a "G" and a "E" that do not sound .
I have other Roland sound modules, and do not experience the same 
thing with them (so I know it is not the A-33 controller ).
I also used another midi keyboard to control the MKS-30 and still 
have the same problem.
 
Has anyone ever experienced this problem, or heard about this 
happening to MKS-30 users ?
Any assistance is greatly appreciated.

Thanks,
The Beatbox Gestapo