Hi 2e0poz,

It's a good suggestion, but the monitor is not on at the moment (see previous post). Neither is the PC. To track the problem down I'm running the driver board from a function generator which is providing square wave signals into the parallel socket on the driver board. Therefore the problem must be with the 33v power supply to the board, the board itself (which is a new replacement), or the stepper motor (of which I have 3 and they all glitch in the same way).

But I think you are right in that there must be some kind of interference from something, or bad earth somewhere.

Hoping to have another go at this after work tonight . . .

Barry