After yet another testdrive last weekend the same problem came back after a while, it worked perfectly for a long time though, so the problem was indeed inside the tank and something blocking the fuelpipe. But now I just needed to get the shit out from the tank somehow. So I bought a magnet and another pump, trying to fish and suck out whatever was inside once again.

First attempt was with the magnet… I don’t claim to be an expert or anything, but I’m pretty sure the fueltank is not supposed to contain rusty nuts and bolts =), so after removing those and then pumping out some more rust particles, I feel more confident that the problem might be solved. A testdrive this weekend will perhaps give the answer.