1Z - Sudden RPM drop during startup

Hello everyone,

I'm stuck on a problem, even though I've been trying my best.
I have both the device and the relevant technical articles, and I am quite proficient in using VAG-COM.

The problem is as follows:

Volkswagen Golf 3 Variant, 1995, 1Z engine, manual transmission.

Approximately 1 second after starting, the idle speed drops very sharply for a short period (down to almost 250 RPM). When the engine is cold, this causes it to stall. When the engine is warm, the problem still occurs, but it is less pronounced, and the engine remains running. Generally, you can only start the engine by using the accelerator pedal.

What I have checked:

Engine mechanics:
- Air in the supply line between the filter and the pump (bubbles longer than 1cm).
--> The engine ran on fuel from a bottle, but the problem still persists.
- Valve timing checked.
--> Nozzle 1. Remove the cylinder, rotate the engine to Top Dead Center (TDC), and ensure the mark on the camshaft is horizontal.
- Vacuum lines checked.
- Solenoid valves inspected.
- Parking boot installed and checked.
- Needle lift injector checked for proper operation while running, and the engine sound changes.
- Mass points are within acceptable limits.
- Timing belt running smoothly, no visible damage (tensioner roller replaced).
- Relay 109 is okay; I also re-soldered it.

Values VAG-COM:

- Idle injection volume: 4.4 mg/H
- Idle speed fluctuates between 880-903 RPM.
- The voltage of the control voltage shifter fluctuates slightly.
- According to the TDI graph, the injection timing is within the tolerance range, but it fluctuates slightly.

The uneven engine speed (as mentioned, approximately 20 RPM difference) is related to these factors.

I can't log the error because it occurs immediately when the system starts.
There are also slight jolts under full load, above 2500 RPM.

My suspicion: Rule-bender.

Therefore, my question is: If I were to temporarily swap the flow meter with one from a different pump, would I need to put the pumps into some kind of default setting or disconnect them? Do you think this is a good idea, or do you have another suggestion?

I'm quite at a loss right now. Thank you for your tips.