After a bit more experimenting the stepper motor driver seems to be the culprit. The decay setting to be specific. I tried rodw's code from the other thread, which BTW I like better, but at first I had the same issue of missed steps. I played around with the decay setting switches and for the most part that solved the problem. I still get the occasional missed step, but it's not as far off as it used to be. The dividing unit is mounted directly to the spindle of a benchtop lathe, so there are only the 12 tooth pulley mounted to the motor and a 72 tooth pulley mounted to the spindle. No flexible couplings or anything like that, so practically there are no backlash issues, but of course there is a bit of flexing from the belt itself. However, in this application it is not an issue. I still have a few stepper drivers that I haven't tried with the new code, and I'm pretty sure one the new drivers will solve the issue for good. I'll just have to wait for my new toys to arrive in the mail. That's my experience as well. I've had these strange issues with two drivers. I suppose you get what you pay for.