If the speed is varying while playing (audibly slowing down), the pot's wiper or resistive element may be dirty. In MM's posted instructions above, I would add step 1.5: Run the adjustment for both 33 and 45 rpm back and forth, from end to end, several times. This will help clean the wiper and resistive track of dirt. Many potentiometers don't have a hard stop at the end of their adjustment, but will make a quiet click, so keep your ears open.