defo an aiming problem, looks like the aiming animator actually jitters, or maybe the beam starts up that fraction before the animator has stopped turning. does it still do it if the beam start time is +1, if it doesnt, adding a quick pause before a beam starts would fix it, this gives the animator time to fully line up on the target.
*edit
in your video, you need to slow it right down and watch the acu and then the beam to see which one is or isnt moving after it starts