Without the platter, the rotor oscillates at high frequency and amplitude while turning at 33rpm. At 78rpm, the oscillations are much less so. With the heavy platter on, oscillations disappear. No gridding noises, etc.
@rays19 The servo 'hunts' without the flywheel effect of the platter. That's why it jerks like that- and many direct drive turntables do exactly the same thing. You got good advice above!