M103 hesitation, narrowing down culprit.

[Sondrusen]

So my 300se with 68.000km on the clock is acting up. The problem is that when the car is idling, It will run OK, but not 100% stable, and will hesitate more and stumble more the warmer the engine is. When cold, the problem is small. But when warm, the car is undrivable.
The symptoms are worst at operating temperature, if you accelerate hard, it will hesitate and stumble, almost like it has some problem ingniting the fuel. That will also happens when you drive, if you keep a steady speed, you can feel the power is being lost slowly. The solution is to let go of the throttle, and press I down again, sometimes that will help to get it up to speed.

I've done a full tune up if the ignition system, replaced and tuned EHA, new O2 sensor, and did the fuel filter a year ago, will change it now, but I don't think the filter is the problem.

After reading online, i narrowed it down to be either the fuel pump, or the lamda adjustment (fuel mixture) or maybe the coil, altough I think that too is unlikely the culprit. Or an electronic problem somewhere.

One thing I have noticed, is when I unplug my temperature sensor at operating temperature, nothing changes to the engine, so I have replaced that too. But it did not help. Can the temperature sensor make my engine act like it does?

What do you guys think can be the culprit?