After reading the entire thread I have a couple thoughts.

Your car has fault codes, they are read though the lambda reading. Fixed value equals a fault, oscillating lambda equals no fault.

There is no normal condition where 1200 RPM idle is correct. The resistor didn't fix anything. At best it masked a problem and caused another. Undo it and start over or you will never fix the problem.

I didn't see confirmation that the air flow pot was adjusted correctly. The low voltage pin must have .7 to .9V when idling with hot engine and no additional load. This has to be right.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff