When you change player's position directly, it messes with the 8direction behavior - the behavior tries to keep up and fix any collision issues. That's why the jittering happens.
You should avoid doing that, use 8direction actions instead - "Set X/Y vector" or "Simulate control".
Also, when you are changing something on every tick, don't forget about delta-time. (This is not needed when using behavior actions)
construct.net/en/tutorials/delta-time-framerate-71