Once again it's readings on the web that gave me most of the answers.
I won't write down here what others have allready done very well.
But if you are interested I can recommend these articles from

To make it short, too much heat from the nozzle is propagating up.
I need to keep my PTFE tube as cool as possible on as long as possible before it enters the hot end.
Let's have a look at what I have in the gar(b)age...

OK.
I have found 8 of these little heat sink for RAM that I have no use of.
I managed to fit 4 of them at the top of the aluminium tube and 3 others just below.
They are just hold in place by themself because they also touch the 3 hexagonal rods, but I added a wire around to fix them thight and to have the best contact I can have between a plane and a cylinder (which means not much).

Heat sink mounted
I think I will try to give them a more roundy shape later.
But for now I hope it will do at least a part of the job...