Why is my feedrate slow with stepper motor

Hello, I'm new to this discussion (any discussion) group so I hope
I'm not asking an ofter repeated question.

I've just created my first CNC router/mill. I used small Oriental
200 set/rev stepper motors 5V 1A. I'm running through a 10pitch screw
and 2:1 reduction - 4000 steps per inch.

I'm driving through a home made driver from some chips I found on the
net for this purpose (Allegro 5804) and K-cam software.

My problem is that as I increase speed, the torque falls off to the
point that motors stall. I've upped the voltage to 12V and this
helped but am still limited to about 2.5 IPM

Is this normal for stepper motors? I'm thinking of designing a
current limiting or chopper(?) circuit but do not know electronics.

I'm guessing others have been here already. Will a chopper circuit
work? could the k-cam software be part of the problem (I get
a "stop" for one cycle every rev that gets more pronounced with speed
rate)

Any comments / advice?

Roger Nicholas