Advice on moving forward

Hi folks, I've spend some bleary hours tolling the archives (LOTS of
good stuff there) but haven't found a directly applicable post so, if
I may ask.

I've spent about 3 months tinkering with stepper drivers designs
based on pic micro's. With acceptable results.
Recently I bought a larger (400oz) motor rated at 3V 4A.
I just can't seem to get this thing to work at more than 150PPS!
At 100PPS it's fine. Really good torque, no problems.

The current setup is a 24V supply into a micro controlled PWM
constant current regulator into a simple, wave drive, controller with
darlington output drivers. Whilst I haven't borrowed the DSO from
work and watched the current across the (unipolar) coils, there is
6600uF on the output of the PWM unit, the DC voltage across the sense
resistor seems rock solid and none of the transistors get more than
warm.

The datasheet for this motor shows a fairly flat torque curve up to
around 1000PPS. Can anyone advise me on where I should start looking
for clues?


Many thanks

PK