Plug in the battery to do a calibration and the Right Rear motor starts to spin approx 1/4 rev then stops, (like it got a current spike) while the flight board is booting . . . after it boots the same motor comes on at maybe half throttle and stays on.
Weird?
Swapped Right Rear and Left Front motors and problem went away.
I can’t imagine how that could have solved problem you mentioned. Perhaps rebooting? Cold solder joint which you temporarily “fixed” by stressing the board in the process of swapping the cables? A drop of dirty water on the PCB which dried out while you were swapping cables? Who knows, but not swapping motors. No way. 
Maybe I’ll swap back and see if prob reappears. Agree with you wholeheartedly, but I rebooted several times in original locations.