Define "crap out".When set to 800 pulse/rev, she will happily move at 2500mm/min. However, when trying to run at 400p/m at 1000, the drivers crap out?
Are the motors stalling, or the drives failing or faulting?
You usually want higher microstepping, as the motors will run smoother. fewer microsteps makes resonance more likely, which can cause stalling.
I have some AM882's, they are near silent, and very smooth. Those motors you have really need a 72V power supply. With a 72V supply, and AM882's, you might see 3x (or more) better performance.