Trouble Running NEMA 23 stepper motors

I'm trying to run a NEMA 23 stepper motor off a mega 2560, ramps 1.4, and an em556s driver. The motor moves but very slowly and in pulses, like it starts and stops a few times per second while moving. I've been looking for what is going wrong for a few months but can't find anything that relates to my situation. I've tried multiple motors and multiple drivers. I don't have all of the specs of the motor with me right now but it is a 4 lead bipolar series stepper if that helps. If anyone has any info that may be of help I'd be very thankful