ok heres what I have done since:

*Got a higher rated Z axis motor-didnt fix the problem.
*swapped z and y drives over and still does it.
*contacted manufacturer and they reckon its some software config issue?? any ideas. The standard software is PCNC which is not very good to me so I havnt tried it but they reckon its perfect with that... IM STUMPED

  ooh and I did try taking putting a cooling fan on drivers and no luck!!!

Seems that he swapped the drives and the problem still exists??

Don't believe it is software config as it does work when it is cooled down, Software is not heat sinsitive(;-)

Doug please clarify  that when you swapped the drives the problem was still on the Zaxis or did it move to the other axis. If it moved to the other axis then I believe you have a drive that fails when it heats up.
(;-) TP

Dougie said that when he swapped the y and z axis cables from the controller to the stepper motors, the problem shifted to the x-axis. 
That implies the driver electronics or software config -  pulse widths, pulse polarity and edge detection.
Or did I mis-read the thread somewhere?

Glad you picked up on that DaveC, the thread seemed to move away from that near the beginning so I assumed I was missing something

Hood