I swapped PCs and now my machine is performing beautifully.
On timed drag-race style runs, both move and jog speeds are accurate and in agreement with indicated settings. In this "unreal" environment, I am getting timed jog speeds of up to 5.25" per minute without missed steps. (I'll be dialing it down for real world use).
FYI the old computer was a Compaq non-Intel 180 MHZ and the new computer is an Acer P200.