Scirocco Register

So they diagnosed that your car idles alright. Great- but that's not the problem?!

Does your throttle open all the way on the pedal? Check cable adjustment...

Double check your cam and ignition timings, set them by the book. Investigate if the centrifugal advance is working correctly (snap-on strobe is very useful) as that would definitely cause sluggish performance like you describe.

Sounds like your fuel pressures are correct. Measure fuel quantity from each injector and check their spray patterns (be safe!). I believe the spec is approx. 50-55ml in 20 seconds from each injector with the air flap at maximum height. The air flap should move freely, that's something to look for too. There is a max flow variance between each injector, sorry can't remember how much

Bon chance

That's a good point. Someone has suggested a rolling road, none near to me though.

Yes throttle opens fully. Injectors are new and flow the same amount in a given time.

Had a look at the crank breather valve. Never given it much though but it is a source of unmetered air, seems to be standard equipment though. Grasping at straws probably..

Honestly, a trip to the rollers would be money well spent! Especially if you find one with a knowledgeable operator.

One more thought before I leave you to it- you already have plenty to chew on- does your fuel pump relay have the fusebox bypass recall done? If not, maybe the pump electrical feed is breaking down (pin A8 overheats), or could be just borderline as the problem just showing up under load. And have you considered the relay itself?
https://workshop-manuals.com/volkswagen ... g__7/82%29

Ps unless the breather is blocked I doubt its influence, unless your engine is really knackered

Thanks for the suggestions.

I have fashioned a relay bypass and have even tried wiring pump direct from switched live and no difference.

Came across a thread a Golf Mk1 website in which someone said he had problems with a pattern injection tank. It appears the car would stop after a few miles although there was plenty of petrol. He replaced with original type tank and no further problems. He thought it was an issue with vacuum or venting on the aftermarket tank.

I did replace the tank during the resto with a 2nd hand replacement - it was solid and clean but had the simple metal swirl pot design so not original. Can't remember where I got it - quite a while ago. I'm not keen on removing the tank - and finding another - but might be worth a try.