Yes, my thinking also - I'm puzzled because the only thing different between not working and working was a setting for Z of 8 & 9 and then back to 6 & 7, as per Angel Tech's suggestion.
And I'm afraid this is always going to be a possibility in this open-ended control system - that's why servos are so much better and expensive.
Again, thanks for your input and time. And yours, Angel Tech.