Yes, its only the user variable jog rate between 'single step' and 'rapid' that uses the pot/pulse circuit. Otherwise the velocity and acceleration is handled by the CPU. Seems an odd fault to occur spontaneously unless it is a aged capacitor.
Definitely worth having the boards out to get eyes on, IMO. It's easy enough but a pain refitting due to space and access restrictions. Unlikely to be contamination but even heavy dust - esp around workshop activity - can upset digital circuits, I've found. I use a dry paintbrush to clean them - although in fact the commonly recommended way of cleaning very dusty old boards is washing with (fairly pure ) water ! Very thorough drying obv.
The timer IC is near the bottom by the amplifier board connectors and the PROMs. Sadly all the board components are inaccessible without removal.