Looking for alternatives to the control unit

Hi,
Okay, I'm having a problem with my Golf (1Z, manufactured in '94). It's shaking, but not like the issues often described at lower RPMs; it happens around 3500 RPM. However, this only occurs during steady driving (using an external cruise control) at a constant speed between 3200 and 3800 RPM. In city traffic, it runs smoothly up to 4500 RPM.
I've checked the engine control unit again. The ESP is now slightly above the ideal line, which seems to be working better. However, it also starts with a bit of reluctance now (before, I only needed to briefly tap the key and the tractor would start).
"Occasionally, the control unit displays '65535' (control unit defect), which usually indicates an implausible brake signal (due to the Winkler T3 conversion without clutch/brake switches or a tachometer signal). Recently, it also showed an implausible speed signal."
Up until recently (until the recurring issue with the KW coilovers in January), the car was running smoothly (even though 90 horsepower should provide a bit more power).
"I also feel that the performance drops significantly above 3000 RPM (the shift quality for turbo seems okay), perhaps it's too much weight for it – those 2 tons."

Since I can't think of any more purely mechanical solutions (I initially considered adjustable engine control or sticking turbo/wastegate actuators, but those seem to be working fine).
I'm considering looking for and trying a different control unit.
Mine has the number 028 906 021 G (Bosch 0281 001 316/317).
The ECU identification number has been changed from AP to G, possibly because it was assembled with a WFS. The (handwritten) coding on the chips is A73BA100 0E00 or B73BA100 E700.
A control unit with the above identifiers appears to be extremely rare; I find many, but all have different identifiers. (for example, the xxx021B seems to be popular...)
Is there a list anywhere, or does anyone have part numbers for engine control units (ECUs) that are compatible with my engine (and immobilizer)? (1Z Golf3, year 1994, originally equipped with a VW WFS engine, apparently retrofitted to the engine).
Thank you for your help.
Marcus.

I don't think your starter motor is defective! It could be related to the mass airflow sensor again, or sometimes a faulty boost pressure regulator.

Hmm.
Initially, I had the same thoughts, which is why I recently checked the wastegate area to make sure everything was working correctly. I replaced the control valve about 20,000 km ago because it was sticking and causing the engine to go into limp mode (though very abruptly when exceeding 3000 rpm - and three VW workshops couldn't find the problem at the time; there was no error code).
But even if it were to occasionally malfunction, the control unit should still switch to emergency mode, and above all: shouldn't it remain in that state until the critical speed for excessive pressure (3000 RPM) is reached?
"If I drive under the same conditions (cruise control, maintaining a consistent speed with a constant engine RPM) but at a higher speed (>3800 RPM), the jerking does not occur."
The LMM values actually look quite good. While browsing, I came across the adaptation setting. Based on the values, it seems like this has already been performed, and the logged values so far all appear to be good and plausible. I'll probably log the data again over a longer stretch of highway. We'll see.

"ECU compatibility seems like a generally interesting topic to me, especially since this question comes up frequently. However, it appears there isn't a comprehensive list (for the FAQ?) of compatible ECUs. Perhaps such a list exists internally at Volkswagen, but even those are likely not very complete – at least based on what I read here about things that actually work, despite Volkswagen saying they aren't compatible."
Hi Marcus,

P.S.: Just a side note: I've now replaced most of the return lines, including the valve on the diesel filter. I've checked all connections for leaks, the diesel filter is new, and the injectors (Bosch) have been inspected and found to be okay.