I took the cover off mine again with the analyser on its side and tried tapping the A22 card all along its edge after removing the perspex cover. I do see some microphony on a 1.1MHz span on 20MHz CF but only enough to make the filter shape wobble slightly. It's a bit like a muted version of what my earlier video showed. But no spikes were visible.
Looking at the circuit near U18 the -15V rail feeding Q17 and U18 has been given some hefty isolation from the rest of the -15V rail using a 10R resistor and a 100uF cap C17. This must be for a reason so it might be worth checking C17 for ESR if you haven't already. It's hard to keep up with what has and hasn't already been tested.
The ribbon cable to connection point J1 near R39 would be a potential weak spot for an intermittent connection. You could try lightly touching this ribbon cable to see if it has developed a poor connection? Obviously, don't overdo this or you could make it unreliable even if it was previously OK. Normally you can diagnose these by lightly wobbling the ribbon up and down a few mm near the connector. Don't twist the ribbon though.
Also, it could be worth inspecting the PCB soldering in this area to look for poor joints as there are several devices on heatsinks. Maybe 25 years of hot/cold cycling has opened up a poor solder joint somewhere in this part of the board.
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can diodes or Zener diodes go funny?
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Yes, and so can transistors. My old HP8405A VVM had several BJTs that went intermittent with temperature. The behaviour became more and more erratic the warmer they got.
The intermittent faults are always the worst!
