I thought about that and tested a range of 180-220C, no cracking, no bubbles, no dirt in the nozzle. I also tested with 2 different materials. No change.
I imagine if it would be a heat thing or systematic extrusion error I would see gaps randomly spaced through the whole print. Not just on long lanes and ever 1.5mm.
So there must be some weird interaction between the lead screw system and the extrusion. I ordered a new lead screw with fine thread but I still have not the slightest idea what the interaction is.