I'm having a real problem with my Z axis. It will travel up without issue, but intermittently will not move down. Here's what I've looked at so far:
1) connections - my molex connections appear solid but I have not checked inside the control box yet. Grounds are connected solidly including Z axis and Y gantry to a common ground point on the controller.
2) Mach3 motor tuning- when I lower acel and or velocity, it seems I have less problem. In fact, when I test only the Z axis (no other axis running) the problem still happens, but much less frequently. When I run a program with X, Y and Z axis in motion at even low acel/vel setting, the problem happens intermittently, but every time i.e. not at the same code location, and not in the same place each time, but each time I try to run, the Z axis fails to move down. Mach3 Z axis DRO shows the Z is moving down, but there is no corresponding movement at the Z axis.

My Z axis is a bit heavy- but moves smoothly and without any sign of mechanical issue. Because the issue is related to not moving DOWN, and up motion seems to be OK, I don't suspect mechanicial issues at this point. I upgraded the Z axis motor to handle the extra weight. I'll get specs on the motor soon.

What should I be checking? What information do you need to help? I use gecko drivers- I'll be looking at this today so I'll post info. I use a controller that runs on my PC parallel port. I use Windows XP, and the machine has NEVER been connected to the internet. Its dedicated to the CNC. I don't even do any design work on it.

I'm wondering if my Z axis motor or its driver has some intermittent issue, or the power supply is not quite adequate when all motors are running.

I'm at a loss right now- I look forward to any help here.