Thought that if you pulled the stem and reset the time, going counter-clockwise, it was supposed to "slow" it down ??? Not sure if that worked or if by how much it would slow clock down . . .

Make sure you have clean points . . . any of the C bodies I have had with the points, had the points dirty and held closed long enough to "burn" the lead wire off - no working clock . . . think you would be better off getting the "newer" quartz conversion. . might also be more accurate - only you would know it is behind the clock face . . . .