I'm not an expert in stepper motors; others here have much more experience than I, but I can try to clarify a few things for you.
24V is generally considered to be low for driving steppers - higher voltage will help "punch through" the inductance of the motors (in layman's terms) to allow for higher speeds and better acceleration without stalling. That being said 24V should normally be sufficient to drive them, just not optimal (you will not even come close to the rated torque value). Our lab product that we manufacture uses 24V (actually, it's 28V now, but used to be 24V) and runs just fine, but our CNC machines use 48V for better torque.
Could you provide the motor inductance? That will help in figuring out the problem. My best guess at this point is the 380oz-in motors may be high inductance, limiting the speed. You said your 180oz-in steppers are being driven fine with the same setup - this points to some difference in the motors - must be inductance, I think. Best to check it first, in any case.
Microstepping is always a good thing - higher setting should help increase max velocity and acceleration as it reduces the affects of resonance which reduces the torque of the motor. More steps = smoother current flow = increased torque = increased speed = better motion.
If anything I've posted here is incorrect or incomplete, other more experienced users will likely jump in. In the meantime, please find out what the inductance of those motors are. That would be the best place to start as your other motors seem to be fine in the same setup. 50rpm would definitely be very slow for a stepper.