Kobi

Just from experience with the 160.
I`ve run across this problem before. The clunk will look fine but it gets worn and cannot keep the fuel flow airless to the pump.
Sometimes just getting down to half tank will cause this problem as well.
Also check the front pulse tube it may have a small hole or slit in the tube from rubbing the cowling and can cause this as well when the fuel demand gets high.

I`ve never had to run a header tank But that also may work if you cant find the problem.
it just sounds like a delivery problem to me,