I have a feeling the idle issue I've been having is caused by two things.
First thing - the air flow potentiometer turns out to be part of the control system for the air idle control valve. If the signal from this potentiometer isn't good then the air idle control valve can't do its job properly.
(Well this is the theory I now have based on bits of reading and much unclearness from the FSM!)
So I removed the fuel distributor assembly from the car (this is not strictly necessary but I wanted to make sure I couldn't see anything else wrong with the sodding thing)
Working on a bench with good lighting is always nice.
I removed the air flow potentiometer and found much like the spare parts I've been playing with (
http://www.peachparts.com/shopforum/tech-help/371777-cis-air-flow-potentiometer-possible-fix.html) that the contact surface of the potentiometer is damaged.
The two little "levers" make contact with the surface of the potentiometer and eventually wear away the surface.
So I've fitted a new one.
It has to be adjusted so that the "computer" gets the correct voltage for the position at idle. There's notes about this here =>
SG-Motorsports Air Flow Meter Potentiometer Replacement
(Found it from this thread =>
M102 8v - Potentiometer fault or Vacuum Leak?)
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Second problem - also idle related - random stalling.
I think this could be due to a poorly adjusted throttle linkage.
The FSM says the linkage should be positioned just so
In the top right hand picture I found there was a gap on the wrong side of part 15 on the car.
So I've changed the position of the upper throttle linkage part by shortening the linkage from the lower throttle plate assembly
(Because this bit is spring loaded)
So that part 15 now has a gap behind it
Hopefully this will stop the indecisiveness at idle which sometimes caused the engine to stall.
Fingers crossed.