i think most on here use a propane torch

run it up to about 3500 and hold it for a few seconds, see if it runs smooth.

Check your vacuum lines. When I used to have CIS, loose or leaky vacuum hoses caused these types of problems.

Update:
I got time to look a bit at the car today...

All the injectors spray. On one of them the shape of the fuel-spray is not very symmetrical at low flow rates - but at higher flow rates the shape gets more symmetrical.
Do I need a new injector?

The pop-off valve is OK.

The sensor plate moves freely.

I unfortunately ran out of time, so I'll test compression and check for vacuum leaks next time I get the chance to work on it.

Update 2:
I finally got time to work on the car today.

Last weekend when I checked the injectors, I did not start the car afterwards.

So today, when I wanted to warm up the engine in preparation for compression test, the engine ran absolutely FINE!

A short test drive confirmed this. :D:D:D

I went along and tested the compression anyway, and the numbers were:

1: 11,0
2: 11,0
3: 11,2
4: 11,0
5: 11,1
6: 10,75

Units in Bar.

Not too bad I guess. :)

My guess is, that the problem was a clogged injector, and that the debris got out before/during the test of the injectors.

Thanks for your help! SmileWavy

Glad it's running fine again.

There's a chance the problem could return. Intermittent problems are usually electrical in nature so that's a possibility if the problem returns. They are usually caused by things like thermal, shorting and or opening.

Thermal malfunctions can occur with things like transistors which are used in the cars CDI. It's not uncommon for for an iffy ignition CD to malfunction when the engine compartment gets hot.

Shorting can be caused by cracked insulation. The engine harness gets baked and the insulation cracks leaving a place for a short to ground to develop.

Opening happen when electrical connectors get corrosion in them interrupting the flow of electricity.

Anyway, good luck and lets hope it doesn't return.