I am having similar troubles. I built some cheap & nasty stepper drives, to see if my machine worked before sinking wads of cash into it. The problem is, you can't achieve any kind of speed without losing torque, unless you have a "chopper" type driver. I think the PWM frequency for steppers is somewhere ~ 20Khz. I'm going to have a bash at building the PIC board at
www.fromorbit.com It looks to be just what I need. Also I'd rather build than buy ready made ones. At least I'll maybe learn something in the process.
Regards Terry...