Bridgeport 720X Heidenhain 2500 Gross Positioning Error X D Help please!

I have an ongoing problem with my Bridgeport 720x with Heidenhain 2500 control. I am getting a gross positioning error X every time I try and reference it. I have tried the following tests:
New ROD encoder. Also swapped Y axis encoder. Fault stayed on X.
New axis drive board (same drive board controls all 3 axis)
Swapped motor with Y axis. Y axis works fine, X still shows the error.
Swapped X & Y encoder plugs on 2500 control and changed parameter 1010 to suit. Error stays on X axis.
Swapped X and Y on drive board. Error stays on X.
I've checked continuity and insulation resistance on both the cables to the motor. No fault showing.
The axis is free to turn by hand.
I have run out of ideas for what to check next and where the fault could lie. My only thought is that the X axis cables have a problem that isn't showing on a meter test as they are the only part that always stays on X.
My tests have eliminated the 2500 control board, axis drive board, motor and encoder. Is there anything I have missed or suggestions on where to try next?
Many thanks
Andy