Stalling 1988 560SL

[BlueNautilus]

I purchased recently an 88 560SL, a time bubble, 27,500 original miles.
Great car, near brand new condition but a problem has come up.
I am lucky to have one of the best MBZ specialists near me and we have approached the issue in the most conservative and methodical fashion, it is still not completely resolved.
When the air temp is very hot (common here in SoCal) the engine will stumble then eventually stall, it happens almost always near idle or very low RPM, it will restart almost right away in most cases.
All the obvious has been checked, vacuum leaks, fuel pumps, etc... We did the trouble shooting by trial and error testing, replacing the reasonably priced parts and swapping good used parts (the expensive ones) for my original parts and checking for the same symptoms.
We swapped the ECU, ignition CU, coolant temperature feeler.
We replaced with original MBZ parts: the overload relay, fuel pump relay, O2 sensor, fuel filter.
We checked the Air flow mass meter sensor, the readings show a messy signal, we replaced the potentiometer with a new one from a brand new old stock box, it was almost impossible to set correctly and get the car to run right so we put the original back on, we had made reference marks on the housing just in case, it was still tricky to adjust correctly but we got the car to run very nicely, we adjusted the mixture to slightly richer from the adjustment screw inside the tower (tube) that runs all the way up through the air filter. The signal reading is a little better but still not clean but I still think we hit a better sweet spot after we reinstalled the original potentiometer. This afternoon, thinking we had isolated the problem we swapped back to my original ignition CU, within a few blocks after I drove away it stalled 3 times and restarted right away :-( Tomorrow we'll install that used ignition CU back on and try to validate that symptom over the next few days.
I hope I don't need to replace the ICU or the whole air flow mass meter as they are very expensive and this is still an almost new car.
If anyone can add anything to help me out I am all ears, many thanks.