Vintage Synth Repair

Hi, I have a  messed up Arp Odyssey 2800 that is in need of some love and repair. 

Currently, I have a laundry list of problems that need to be addressed.  

1.) ADSR is not working at all but AR works OK.

2.) S&H is not working at all.

3.) The transpose switch is not working at highest octave setting for oscillator 2.

4.) Osc 1 & 2 are unstable at lower frequencies but seem to be stable at high frequencies.

Here is what I have fixed or replaced so far:

-I just rebuilt the power supply and replaced all of the tantalum throughout the synth. 

-I have replaced the sliders for the fine and coarse tune for both oscillators.

-I have replace all of the sliders for the ADSR section.

-I have tested all switches and sliders and they seem to be functioning properly now.

-I replaced the two 4011 cmos chips in the oscillators.

Most of the other things don't bother me that much and I shouldn't have that much trouble finding the faults and fixing them.  What I really need help with is this darned instability in the oscillators.  Are there common faults that often display the symptoms I have described?  What is a good troubleshooting sequence?  My intuition is telling me that the instability has something to do with capacitive reactance?

-Mike

Some of this seems to point at Board A (octave switch and and possibly the
unstable behaviour).

I tend to exchange all opamps from Board A (with the same type). In the
past I've witnessed improvements several times; probably the manufacturing
quality of even generic opamps is higher today than 40 years ago.

Also clean Board A very carefully. Board A is notorius for instability when
contaminated.