Generally, the drivertest will show either excellent, or fail. So excellent is not always excellent?Also the Mach3 Driver test shows excellent output.
Run it again, and see if it's a perfectly flat line. Also watch the pulse rate and see how much it varies. A really good PC won't vary by more than a few pulses. If it's more than 100 it's marginal.
What are your pulse widths set to in Mach3? Try setting them to 15 and see if it helps?
Are you running a direct drive to the belts?
If you run the machine at 50ipm, does it still do the same thing?
Those motors will not run very fast on 36V. They really need 100V or more to get the best performance from them.