Trouble jogging Y

Took a few evenings, but got everything assembled -
Downloaded GSender as instructions said, but jogging Y is pretty choppy.
I set the distance to 300mm, but it only goes like a little ways, and if I click GSender Y- too quickly, it doesn’t move at all, just grinds.
I’ve checked the DIP switches, the Couplings, I can rotate the rods by hand, machine is square (and clamped down), I’ve reseated the plugs at the longboard, and at the motors, I moved the gantry all the way back, so it’s square against the Y rails too.

This same thing also seems to happen using easel (what I was using with my smaller 3018)
(cross posted to FB with video - Redirecting...)

Thanks, I appreciate how clear the instructions were, with big pictures and markings where to connect things…