To comment on both your points:

Assuming you've got best quality HT components now, all should be well there. Did you replace the shield behind the rotor arm? To recap, the distributor cap, rotor and shield are rather expensive but I bear in mind that my originals lasted 19 years and were still working so I can't really complain.

For cap and rotor, MB ONLY unless cash is very tight in which case Bosch/Beru MAY be OK - but ONLY Bosch/Beru.

I now have all MB aside from HT leads that are Beru - after 5 years all seems well.

Also, non-resistor plugs are the best choice. Most aftermarket websites suggest resistor types by default. I use Bosch (very rare now), Beru or NGK. These are the cheap, copper-core plugs.

Continued hot idle problems may be the injectors leaking, especially if any misfire clears after a minute. In my case, the hot start would lead to misfiring until the leaked fuel had burnt off; thereafter OK. So, new injectors. My M104 engine demanded injectors of about double the price of the earlier M103 injectors. So, I parted with about €360 for the injectors and, with the seals and guides, €500 in total. Not cheap but the end result was most satisfying.

Also, make sure your Idle Control Valve is operating correctly. If it doesn't close properly, it will introduce un-metered air and that's bad as we've already discussed. Some people report that you can flush out the ICV with WD40, petrol or other solvents and all is well. Worth doing.

Once you have eliminated all the problems we've now discussed, make sure your temperature sensors are good by measuring the resistance at cold, medium and hot and comparing those values against a chart that you should be able to find somewhere on the Interweb. You have a coolant temperature sensor that sends signals to the engine ECU. You may have an air intake temperature sensor. Both these influence fuel mixture so make sure these are good before attempting to adjust duty cycle.

Buy a multimeter with Duty Cycle as this makes things simpler.

Adjust Duty Cycle to around 50%, as recommended. Read up on this because the procedure will depend on your car: Kat/non-Kat etc.

In my case, I set the Duty Cycle a bit higher than 50% as this made everything more responsive. In theory, the Duty Cycle should change by no more than 10% between idle and 2000 rpm. However, in my case, it rises from about 55% to over 65% - more like 20%. Having said that, the car goes really well so who cares.

As has been said already, these cars are old and certain adjustments are sometimes fiddled with to compensate for out-of-spec components elsewhere. I'm sure that's the case with mine. A good example of things being a bit awry on mine is that I get about 35 mpg on a motorway dash of 500 km. Pretty good. Around Paris, with endless stop/start, about 14 mpg. Not good. Maybe if all associated components were new that would change.

Bonne chance.

RayH