Usually when stepper motors will only go in one direction, there turns out to be a wiring problem to account for that. Is that happening with all your motors, or just some of them? Stalling issues are a lot more common with steppers; often they can be cured by reducing the acceleration value significantly. Try cutting it in half and seeing if things improve. Another thing that can cause steppers to perform poorly at high speeds is a mismatch between a motor's inductance and the power being fed to it. How much inductance do those motors have, and how many volts are you giving them?