I've watched the displays and this is what happens: As long as there is only one motion command per line everything is fine. Then when it starts with two motion commands the first such line tries to run at 33.3 IPM instead of the 18 everything is set for and even if I put F18 multiple times in the GCode including just prior to the first line with multiple motions. The next lines then go at 18 IPM but by then it is too late as the X axis stalled on the attempt at 33 IPM and everything is screwed up. Just to be clear, I had none of these problems running the same machine with the parallel port. On my machine the X axis is the main one and so the X and Z work together during cuts. I have a table full of successful test projects done with the parallel port set up so I know my machine works.