Bridgeport Interact Series 1 strange X-axis gross positioning error

I finally got around to starting to recommission my Bridgeport Interact Series 1 with a TNC 145 controller today and found a couple of issues. Firstly, I need to new monitor as the present one is unreadable in parts due to the glass being sort of hazy. I just need to decide whether to shoehorn in the large monitor I have that's compatible with both the TNC145 and TNC151B or bite the bullet and buy a straight forward plug and play one. Secondly, the axis are homing fine but the numbers being displayed in reference mode for the homed positions do not tie up with those in the maintenance manual for the machine, so I suspect that parameters 31 to 36 are suspect but didn't want to start messing with them straight away. Whilst the first two issues are relatively easy to fix I have an intriguing issue with the X-axis. After homing, if I try to use rapid traverse with the joystick it will go 10 to 20mm and then stop and "gross positioning error" comes up, well that's at least what I have deduced it is saying. However, if I use the handwheel after homing I can traverse the axis all the way across, which would appear to rule out an encoder issue. Could it be a loose belt that's slipping with the rapid traverse on the joystick but is ok with the slower traverse with the handwheel or could it be parameter 0 is set too high and the machine can't cope, or is there something else?
Thanks
Andy