This problem is now solved, it was after all dirt inside the tank!
I want to inform you about what happened so others can skip the trouble I had. It did not seem logic to me at first but now I know better... So as I wrote earlier, when I got the second-hand tank home I thought I'd better flush it before installing it, and this I did by filling it with 10 liters of fuel, through the inlet hose (tha large pipe), stirring around, and emptying it again through the same inlet hose. I did this at least twice, basically no dirt came out.
But after all the trouble I got and your tips, I finally removed the tank again and loosened the upper cap for the fuel meter instead. Under this cap, on top of the tank, there is a large hole that I emptied the fuel through. And this time a lot of stuff came out that shouldn't have been there!!! So apparently the former owner of the tank had used silocone to tighten the upper cap, and large chunks of silicone had fallen down into the tank. When I emptied the fuel thought the inlet hose, this stuff did not come out because the inlet pipe continues about one inch inside the tank. I did not know this until I saw it from above... so I never got out these chunks before.
Clearly Ack's theory was correct, some pieces of dirt got sucked onto the pipe that would take out the fuel to the fuel filter, on the bottom of the tank, causing the engine to stall. When resting for a while, it ran again until driving a while etc...
So my nr.1 tip for anyone who installs a used fuel tank would be to make sure you clean and flusgh it through the upper hole meant for the fuel meter, not though the inlet hose!
When you buy a new tank this is normally not a problem but in my used tank the fuel meter was installed already so I never gave that a thought at first.
I anyway decided to install a new tank at last because when this first problem was solved, I discovered more rust and leaking also on this second hand tank, so I was fed up and found a new tank instead made in Canada, aluminium one very nice and clean at a good price too.
Tanks for all the help, now the car runs like a champ again!! My next project would be to install Calmini's 5 inch suspension and look for 31 inch tyres