SOTA Sapphire speed issue

@effischer 
Thanks very much for the extremely helpful post!  
I did remove the speed control cover to have a look around. I noticed that the belt sits at the bottom of the convex pulley when the motor is off. When turned on, it "rides up" toward the center. Is this normal behavior? Also, I don't think it's exactly centered... looks a bit low to me. 
Thanks again!

rebbi, From what you describe, it sounds as though the belt is a bit looser than intended.  Ergo, the belt may be slipping momentarily at start up. In your original post, you do not mention that SOTA replaced the belt when they performed the previous service.  I'd do that now and see what happens. My bet is that the problem will be ameliorated.

You've likely discovered your problem, rabbi.  The belt should remain in a fixed position on the pulley regardless of whether the motor is running.  I suspect you may have a loose/failed motor mount.  It's possible you may have a failed motor bearing, but that would almost certainly be audible.  The best course of action would be to call Donna and see what she might suggest.  I'm not familiar enough with the motor assembly to give you a good opinion about a DIY attempt.  BTW, the belt should be centered on the pulley when everything is level and adjusted correctly.

For what it's worth, if the belt is riding low on the pulley, and if the pulley is narrowest at its mid-point, then the speed of the platter would tend to be a bit faster than correct, assuming the pulley is rotating at its intended velocity.

@lewm Thank you very much for chiming in. Actually, the pulley in profile is convex – its midpoint is the fattest part. For whatever that's worth. ;-)