Control bipolar stepper motor with potentiometer?

I don't think the speed call has any effect when you are only stepping the motor by one step at a time. Try stepping it two and see if that makes the difference. But if the motor is skipping then you are asking it to go faster than it can.

scullies:   digitalWrite(COIL1,LOW);   digitalWrite(COIL2,LOW);   digitalWrite(COIL3,LOW);   digitalWrite(COIL4,LOW);

Are you powering down your stepper as soon as you think it is in the right place? Why are you doing that?

Thanks for the replies guys, I have been messing around with the speed and stepping, I also swapped the +5v and ground around to use the other end of the pot range. All that has helped a lot,

I was shutting it down as it was getting so hot, I couldnt touch it. Its rated up to 36v and 3 amp so I used a 12v 2 amp supply which was making things to hot. I have changed to a 5v 450mA now and its cool as ice. I was using laptop chargers that I have lying around the house.

Still interested if a rotary switch will work to control the stepper. My understanding is that the Arduino takes the different voltage readings from the pot and this determins the stepper motor opertaion. Would a rotary switch also put out different voltages as the switch is turned?

I also swapped the +5v and ground around to use the other end of the pot range.

That sounds like you have a log pot not a linear one.

Would a rotary switch also put out different voltages as the switch is turned?

No.