I still had this outstanding issue which I encountered last year when I took the car out for picking up a set of wheels. At that small trip the engine was playing up occassionly. I was unsure regarding the exact cause as it came out of the blue. Prior to this I had droven the car long distance across Europe and parked it afterwards in good condition. Except for the leakage as stated in this blog: http://erwin400.blogspot.nl/2018/01/gas-leak-repair.html
As yesterday I mounted the refurbished wheels and the weather was nice I had another occassion to make a little test trip. Clearly the leakage was fixed but the engine playing up was still there. It manifested mostly when driving above 100 km/h but sometimes also at slower speeds. Still the engine runs so nice and smooth. I suspected the carburettors potentially needs tweaking or got dirty somehow. So afterwards I parked the car in the garage and had another look under the bonnet. Visually all looked good. So I just checked the large wing screw on the carburettors. And one of them was very loose, so I tighten it up. And also firmly tighten the other ones. I could not bother to get the car out again. And since I was around today I had time to check. So I took it out for another test drive. And guess what? It's fixed! I guess it sucked extra air via this "leak" when speeding and as such the gas mixture is off-set.
So todays top tip for 400 carb owners. If your engine is stalling / holding up at higher speeds, check the wing screws on top of your Webers.