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If this were a piston engined car I would be diving for the auxilary air valve, I have no idea about your turbo / fuel injection system but if it has such a thing I would be looking at it and its control, also with a fuel injection system there will be a temperature sender for the ecu, worth checking.

Sticking throttle shaft or throttle position sensor?

Hum, OK I'll try and take a look at a possible sensor failing... Seems like I'll struggle a little for that lol.
Logically, where would one be positioned?
And I don't think it has to do with a throw problem. Idling being high only, it could be. But here it's fluctuating like a cammed v8 so I doubt it has much to do with that.

I've asked around. Showed the video to Efi technician. He suggests fault with idle air control valve or vac leak at inlet manifold.

Awesome, thanks man! I'll go look at it a bit deeply this week end. I'll keep this thread updated with my progress!
I can only hope it's not a vacuum leak. Those 80's engines are a bit of a mess with vacuum hoses lol

You can check for a Vacuum leak by spraying a solvent based spray eg WD 40 around the hoses, you will hear the revs change if there is.

It could be that the thottle position sensor is out of adjustment or simply worn because of the high age or it might be a vacuum leak. I don't think the idle control valve is the cause because it looks like it is working but it's trying to adjust to something which shouldn't be there.

Noticed I never updated this... I found the culprit!
Believe it or not, but my A/C was responsible for that haha... The relay of the solenoid that raises the idling when the AC is on. No more problems now and it idles very smoothly