740 poor running and stalling

[Beadybc]

Hello here's a video for you to watch that shows a typical 10 minute stint of the car running:

https://www.youtube.com/watch?v=5kKeaRPm-I8

I have also attached some photos.

The car is running poorly, sometimes on three cylinders and most of the time not in a smooth manner causing the car to shake. Sometimes as you can see in the video, any level of input on the throttle causes the revs to drop dangerously low before they then go to where they should be however a few minutes later the car will respond normally to throttle input as long as you don't go over 20% throttle, its very on and off. I have noticed that as the car warms up the revs will drop below the 900rpm it's supposed to stay at but adjusting the idle screw makes no difference and adjusting the timing just lowers it even further.

Once it has warmed up, the big problem is letting off of the throttle. If I can rev the engine, as soon as I let off the throttle, the revs drop and it stalls (again this can be seen in the video). Also visible in the video is the fact that once it has warmed up and then stalls, starting it back up is impossible however after say an hour turned off it will start again fine.

Things that have been done so far:
-Cleaned throttle body
-Correctly adjusted the throttle position
-Replaced the Fuel: In tank pump, Main Pump and Filter. All are functioning.
-Checked and it is definitely getting sparks from all plugs
-Flushed out coolant system and replaced coolant
-Replaced gaskets around the inlet system
-Checked for accessible vacuum leaks using wd40 whilst it is running, found nothing

Things that are probably wrong but haven't been fixed yet:
-You will hear in the video that the water pump is making unhappy noises but only after about 1.5k rpm

-Now that I have listened to the car through headphones I can hear what might be a knocking noise coming from the engine after I let off the throttle.

-The battery was flat when I picked the car up so we charged it, the sides of the battery did bow out slightly whilst doing so.

-I've also read that a dead Idle Control Valve/ Air Auxiliary Valve could be the cause so I connected it directly to the battery and it did eventually close however I'm not actually sure how it is supposed to function so if anyone knows how it would be useful.

-When I picked up the car brambles has gotten into the engine bay and into the vent in the photos, my biggest fear is that one of them got behind the firewall and punctured something there that is causing a massive vacuum leak. Is there anything that goes into the firewall that could cause this much damage if punctured and where should I test vacuum pressure on this engine?

Could any/all of these be contributors?

The car is a 1989 740 Wagon B200E 2L

Any help is appreciated.