It looks as if there's a small rotary switch on the side of each driver that allows you to set the peak motor current. Perhaps you could try a lower setting. In broad terms, the current limits the torque, whereas the voltage (36V?) limits the max speed.

Some switching PSUs will briefly shut down and restart if they see an overcurrent spike, so it's possible this is happening. You might find that fitting a large capacitor on the output of the PSU will smooth out any transient current spikes. If I'm right in thinking it has a hiccup mode overcurrent protection, it may then take a few attempts to establish the output but may be less immune to dropping out if that's what's happening.