I forgot to give feedback about the cause.
While at the VP dealer for the starter repair i inquired about the cooling
problem. They had absolutely no clue but advised not to pull the pump without new gaskets at hand.
So i ordered new pump plus gaskets. When these finally arrived i pulled the pump and guess what... the old pump was in excellent shape.
Between the pump and the engine block there is a 3mm spacer and two thick and quite complicate paper gaskets.
One of these gaskets had a void between the suction and the pressure side, thus creating a shortcut that reduced pressure and output.
I guess the previous owner pulled the pump for some reason and did not replace the gaskets.
With new gaskets the engine has been working like a charm for a few days now. Luckily i could return the new pump, so the problem cost me 30 euro for the gaskets (plus 80 euro s&h)