Our little wood router table running Mach3  running version R2.0.026   had a just completed a cut on a foam mold, and instead of returning to  Z 0.0,  tried to return to Z 58.0.   
Blew past the limit switch and pegged itself against the motor mount.

This is an old router table I bought from a custom guitar maker in Monterey Ca,  and it came with it's own computer and Mach 3 software.
It's worked really well for the last 10 years or so.
Just today, it acts up.

We've tried several things, including a restart, cleaning out the motor control boxes, and when we power up the router and Mach 3, 
I can move x and y axis smoothly with the keyboard jog,  but when I try to move the Z downward it will move about 1/4 inch, and then move back up and crash the motor mount again. 

I've tried to hold the down key done continuously and it will move in 1/4 inch increments downward with harsh stalls between each move, and after 2 inches,  will recycle up to the motor mount again even with my finger on the down button. 

I've even moved the Z down about 2 inches and killed power to the router, holding the Z in place.   But when I power back up, and power up Mach 3,  as soon as hit a reset button in Mach 3, the Z axis takes off upward and pegs the motor mount once more.  And the power to the stepper motor doesn't cut, so the motor gets really hot quickly. 

Any ideas?  Thanks for your patience with a newb.

Could be a loose wire on the direction pin.

Thanks  Gerry.
Could that be on the stepper  motor side connector, or down in the control box, or either?

No, it would be between the PC and the drive, not at the motor.

Once again,  thanks for the flashlight in the dark!  Will look at the control box where the parallel cable comes in from the PC.