I have this problem whenever I am trying to really work the motor. For example: going up a steep hill without peddling, holding the brakes and activating the throttle. I think it's a cutoff feature in either the battery management or the controller. It will cut out a couple times and then completely shut down.
It is probably a good thing because going up a hill without peddling heats everything up a lot. Using a 48 volt battery may trigger the same at starts on flat ground.