81 EG engine cough and cutting out when in traffic.

So my Golf developed an annoying intermitant problem when I took it for a week away.

It has been running really well recently, and had a full service during lockdown, less than 500 miles ago. Its an 81 GLi, so 1.6 EG lump thats standard.

Bear with me, i'll explain the events!

The issue first appeared in stop start queing traffic,  with the engine hot, but not yet hot enough for the fan to cut in. I changed down a gear and put my foot down to have the engine cut out, coasted onto the grass, it turned over for quite a while until it spluttered into life, then once running it revved perfectly. I checked all the connections under then bonnet at this stage and looked for anything obviously loose, leaking, dirty etc.

Further on in more traffic a bad cough appeared when accelerating or cruising, again with the engine over normal running temp. I stopped to let it cool for 30 mins, and removed and refitted connections etc under the bonnet.
From then on i ran the heater full blast to keep it a bit cooler, the last stretch to the campsite it was coughing quite regularly.

Having not used it for a couple of days i took it to eurocarparts to get a replacement coil, dizzy cap and rotor arm. When i came back out it wouldn't immediately start, so i changed the coil, it started, turned it off to pack the tools away, then it wouldn't start. Fitted the dizzy cap and rotor arm and it started, i didn't turn it off. We drove it around a bit then and it performed faultlessly, including turning off and starting a few times.

It was then fine for about 100 miles, until we were queing in traffic on the way home where it coughed again twice like it had done on our drive away!
Its since been put in its garage and not touched for two weeks in disgust When i serviced it over lockdown, i did not replace the dizzy cap and rotor arm (short sight on my part), it had a new hall sensor 3 years ago (JP group part from Heritage).
The only parts of the ignition system that haven't been replaced in my ownership are the TCI and the wiring loom.

Anyone got any ideas? I'm thinking this is ignition related but not 100% sure. (sorry for the essay!).