Okay so big big big update!
I built my own smoke tester and did a smoke test. No vacuum leaks! I use a test port on the intake boot to bring the smoke into the intake system, when reinstalling the plug in the test port I used some sealant and after doing this the idle rpm dropped so I guess there was a vacuum leak there.
But the engine still idled very rough. It constantly dropped rpms, then the ECU kicks in and brings the rpms back up. This repeats itself.
BUT! all of a sudden this was gone, the engine idled around 950rpms and started smoothly. It did hold back above 4000rpm when driving. When I went to adjust the ignition I noticed the ignition was severly retarded, and when I turned the distributor to bring it back it would retard again after a while. This led me to the knock sensor, which was bad. I replaced this (and torqued it down properly) and now the ignition was adjustable. I brought the engine up to temperature, connected my multimeter and stroboscope and got adjusting. I got the DPR current to 'search' between 4 and 6 mA, and the ignition to 6° before TDC. Idle rpms where too high now, but I brought these down by adjusting the idle air bypass screw. After every adjustment of this screw I checked and adjusted mixture and ignition again.
Finally I got the idle rpms to sit around 950rpm, the mixture at 5mA +/-1 and the ignition at 6° before TDC. The exact specs as given by the Bentley manual. Throttle respons was great. I tightend down the distributor, checked the ignition one last time and removed all the test equipement. I shut off the engine.
I cleared out my driveway with the intention of doing a test drive, so half an hour later I come back to the car to do a test drive. All of a sudden the car is very hard to start, and when it starts the idle problem is back. Throttle response is still great, but below about 2000rpm the car runs extremely rough. The problem doesn't go away when the engine warms up, oil temp was 96°C so the engine was warm.
I did an electrical check of the knock sensor system as explained in the Bentley manual and everything came back fine. Hall sensor on the distributor recieves power, and the ignition module is fine according to the electrical tests in the Bentley manual. There was some oil in the hall sensor connector, but cleaning this out did not eliminate the problem.
What am I missing???