While you have everything apart check the Speed Control Assembly. Specifically the teeth on the porkchop and worm gear (see arrows in the picture below). Also check the teeth on the speed control dial, the speed control idler gear and the back side of the speed control handle. If the previous owner tried to change speeds with the pulley sheave frozen in place it may have damaged teeth on the speed controller assembly. If they are fine than great if not they can easily be replaced. While you have it apart check all the headstock parts for wear and damage. If any parts have pitting from rust consider if they need to be replaced or will they still work smoothly as they function.
Mark V completely upgraded to Mark 7
Mark V 520
All SPT's & 2 Power Stations
Model 10ER S/N R64000 first one I restored on bench w/ metal ends & retractable casters. Has Speed Changer, Model 4E Jointer, Jig Saw with lamp, a complete set of original accessories & much more.
Model 10E S/N 1077 oldest one I have restored. On bench w/ metal ends & retractable casters.