Peco Frizinghall turntable

I have installed the Peco OO turntable with the Frizinghall motor kit. There have been other threads about this combination but not mentioning this particular problem as far as I am aware.
The motor is driven from the same Gaugemaster controller as the track, via a DPDT centre-off switch
It works (subject to installing the polarity switch) but the rotation is not smooth; the turntable judders. The worm is not jammed against the large gear:





so I can't see why this is happening.
The one design fault I find is that having cemented the securing collar underneath, one can not get to the central spindle for maintenance or adjustment.
Can anyone advise? Thanks. Colin