It may be a bit of a long shot, but you might want to also check the actual temperature of the hot end tip as opposed to the reported temperature.
If the extruder feed rate is slower then the filament will have more time to liquify as it passes through.
Once corrected for motion it may not be able to heat the filament fast enough or may have too much resistance because the filament will not be liquid enough.
If your step stick voltage is set correctly and there is nothing dragging on the filament and there are no nozzle obstructions, and it still slips/skips, that could be the problem.
I know mine was running much colder than was being reported until I discovered the problem and corrected it by choosing a different thermistor curve.
It was type1 by default, now I have it set to 5 (around line 195 in Configuration.h).
It would have been nice if the firmware had been pre-installed and configured as claimed in the build manual, but it wasn't.
Particularly since you just installed firmware, don't assume it is correct.