Jazz, using my computer isn't really an option as it's built in to the machines old control box BUT I did use his computer to run my machine to eliminate the PP & the software, worked no problem at all. I also tried my serial cable on his machine to rule that out lol, Plus as the seller has now sent him 3 different bobs he has 3 cables of his own that we have tried as well.

Eddy, I've tuned the motors & backed them off far enough from their maximum that it shouldn't be a problem but the machine was doing it before I tuned the motors & the initial setting they give you to plug in to mach3 are so slow I can't see it being a factor. Compatibility I haven't got a clue about if I'm being honest but all 3 motors are the same & only 2 axis seem to have this problem. Jazz is right there are no safety cut outs with this stuff lol

Clive, it had crossed my mind that it might be down to heat in some way but wasn't sure what to do to try & eliminate anything like that, will try a couple of fans blowing on them.

Thanks for the help & suggestions people, gives me something else to try.