Like you I am having trouble with an Estarli e20.7 seat post as well. On a poor road surface the seat post slowly slips down, but to pull it out I have to wiggle the seat back and forth while lifting so hard the back wheel lifts up.
I am also less than happy that at 15.5mph ALL the power is cut then a couple of seconds later the speed builds up again. Repeat every few seconds. Dropping an assistance level or two reduces the jerking but gradients change every few seconds here so maintaining 15.5 is not easy.
Any solutions?
Thanks