DB25 cable strange problem

Anyone know why when using a new DB25 cable (3 metres), to extend the distance from computer to controller one of my motors runs faster and further? I have a gantry table with XYYZZ and when I use the cable one of the Z motors runs much faster and further than the other. The other motors all seem to be running normally. If I remove the cable and connect directly to the onboard controller, with the short ribbon cable, all works normally. 
I am using a Mach3 5 Axis BOB


 

Do you use individual DIR and STEP signals for each motor? If so, you need 10 output pins. Thus, apparently one Z motor is connected to outputs other than D0-D7. Try connecting all fast STEP signals to D0-D7, and connect slow DIR signals to the other pins.

are D0-D7 designation on the DB25 cable? any idea what they equate to on the BOB or is that something I set in Linuxcnc