my old mark VII did the same thing until I did " the fix" a couple years ago and it has worked great ever since.
There is too much slop between the snap ring and the speed control knob to hold the spring loaded balls snug enough in their detents to "hold" a set speed.
If I remember correctly I; removed the snap ring and threw it away;
drilled and tapped the shaft for the speed cam for an 8-32 screw;
put the shaft into the speed control knob,put pressure on it so it bottomed out against the back of the control to see how much of the shaft sticks out above the control knob (where the snap ring groove is; I think it stuck out approx. .100"). then I got a washer that was slightly thicker than the height of the protruding shaft as a spacer. then a small washer for the 8-32 screw but with OD large enough to span the shaft OD (3/8"?) and some of the spacer washer.
Then tightened up the screw to remove the slop in the control knob.
SS Mark VII(sn 405025), SSband saw, SS 4" jointer, SS Mark V 1980,
Smithy SuperShop 720, Craftsman RAS, Ridgid TS2412 Table Saw,
Delta 12" planer