After weeks or troubleshooting and tons of support from Artruro Duncan, I have my lathe working, mostly.  Just one small issue I need some help with.

I can jog both axes using continuous mode and the X and Z + and - keys.  It works flawlessly.

But then I switch to MPG mode and move the handwheel even one click in either direction, the axis will run away.  I can't stop the motion except by hitting the estop.

What am I missing?

Huge shoutout to Artruro, BTW!  He has been great.

We need more details about the software and the control.

Graham,
This is my setup, please let me know if there is something else that would be helpful.  All of the boards and the MPG came from CNC4PC:
PC running Windows 7
2 Startech PCI parallel port cards.
Port 1 is connected to a C10 BOB which is in turn connected to two DM556 stepper drivers
I have two home switches wired in parallel connected to pin 15 on the C10.
Port 2 is connected to a C22 board, which is in turn connected to an MPG2 pendant.
In Mach3 I enabled port 2 and set pins 2-9 as inputs

Have you followed this :-

https://cnc4pc.com/blog/post/setting-up-mpg234-over-lpt2-using-mach3

yes.   I think I know what is going on and I believe it is a problem with the parallel port card.  They have been troublesome all along.  As of this moment, and it could change, I believe I will buy a UC400ETH from CNCDrive and use that in place of the parellel cards.

upon further investigation it looks like the UC400ETH won't work for me because both ports are configured to be primarily output signals.  Instead I am looking into a Warp9 ESS, specifically the B-ESS.