Hi, Dennis
Mach's default jog speed (Arrow Keys) is about F6, 6 in a min., A G00 will run at your max Velocity setting speed.
Try a G01 Z-1, Maybe thats what's going on hear, But the run-away issue must be your Step's Per aren't set right.
I've sent you a Personal Msg.
Post your XMl File as Hood Asked, It's in your Mach3 Folder. C:\Mach3\Mach3Turn.XML, or where ever Mach3 is install on your computer.
Sometimes the install to Dir is Mach3R183.....?, If you didn't change it at Select Destination Directory window during your install
Thanks, Chip