Those
rapid fire retractions might be just too much pressure in the melt chamber, causing the extruder stepper to be pushed backwards?
I had the same problem, blaming my 0.9° steppers. I print on LokBuild now and use default nozzle width setting for 1st layer.
Another way of dealing with it, is reducing extruder stepper current. It will skip steps instead of pushing to hard until it fails.