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OK, sounds like you have covered the engine and fuel basics, and more.
Since there is never a problem when the AC is off (never, right?), perhaps something related to the AC system occasionally has a different load to the engine, or an or electrical change.
So let's take a different direction:
1. Is the system R12, or converted to 134a? Professionally converted or DIY? Looking to see if the system is overcharged and causes compressor stall when engaging.
2. When the AC has been on for a while what are the vent and ambient temps? High pressures can create heavier load on compressor when the clutch kicks in.
3. Do you have a gauge system to measure the suction/discharge pressures on the AC compressor when the idle speed drops? If so, can you collect some data about the pressures vs. the idle speed drop? Once again looking for high pressures.
4. With the engine idling, does the idle speed drop when the AC compressor kicks in? Does the idle speed change every time, or intermittently?
5. Really reaching here, but if you turn the AC on from the dash to engage AC compressor clutch while closing the throttle, does the compressor clutch come on and the idle drops low at the same time?
6. Same test, but if you have the belt off the AC compressor and turn the AC on from the dash while letting up on the throttle, does the compressor clutch come on and the idle drops low?
For #5 & 6: Looking to see if the clutch current causes the alternator or battery voltage to drop, caused by resistance in a ground or 12V power somewhere. You could also check the battery voltage at the DME, but measure it on the AC volts scale since it will be a short lived transient.)
I'll think about it some more.........
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