I borrowed a function generator from work yesterday and set it up for a square wave TTL output of 8KHz (=1500mm/min on my system). I connected this to pin3 and earth on the driver board and the stepper motor ran smoothly. I then tried different speeds and at low speeds <8KHz the motors glitched. This was the same result as when the PC is connected whereby it glitched at lower speeds. Although this doesn't rule out the PC glitching, it certainly means that the driver board, stepper motors, or power supply are causing part of the problem and need to be resolved before connecting it back to the computer.

The driver board is a brand new replacement since the previous board glitched (but on the X axis only). The power supply is a 33v 10A unit and is the one recommended by Roy at DIY CNC. This power supply provides 33v (I've checked this with a DVM), which goes through a switch and a current display 0-5A. I plan to directly connect the power to the board to elimate the switch and meter. Next is to double check the wiring to the steppers, but I was very careful with this when I wired them up. Might be getting somewhere !!