I initially thought along the same lines Tweakie, thanks for the suggestion anyway. As I said it doesn't really matter now, but I'd like to know what caused it, if only for academic reasons. The strangest thing about the problem was that, although I didn't mention it in my last post, I fully reinstalled a fresh copy of M3 then used an XML file from a backup that I knew was working with the VFD in question. I suspected it was a wiring issue, but I wasn't prepared to fault find it any further.
Another strange thing was that it never reared it's ugly head when running a cycle, it only failed after a cycle had finished.
My theory, for what it's worth, is that the disable line on the Gecko was close to disabling the drive, and did so occasionally, but only kicked in after use, since, during a cycle the load on the power supply was sufficient to keep the voltage below the TTL threshold. We will however, never know.
Dave