I am late with my reply, sorry for that. I just wanted to find out what was wrong with it before responding here. After my first symptoms I decided to turn the idle adjustment knob in the base idle adjustment setting really quick and set the RPM to 700-750 (can't remember exactly). The problems were less frequent and it ran acceptable. I changed the lambda, because I didn't trust him, but it didn't change the behaviour. Bosch says you should replace it every 100-150k KM (car has done 220k now), so I thought it would be a good thing to try. If it didn't help, I had at least performed proper maintenance.
This week the car really started to give up. I even lost a cylinder! That gave me a helpful clue though; ignition? Injection? Throttle body? I got a replacement throttle body just to see if the problem would be in there. Taking the intake filter box off, I decided to test the injectors (measure the resistance of all injectors, should be 11,5 ohms) and number 4 proved broken (resistance was infinite). So I guess the problem (could be) solved. In the coming week the new (used) injector rails will come in (they only sold a full rail, paid 35 euros so no problem) and I will replace it. I think the injector was giving up intermittently (those problems are the most difficult to solve), glad it gave up completely so I can finally fix it.
I'll follow this up when It has been replaced, thank you for playing.
