Here's the progress



Careful study has revealed this is a backlash setting problem of some sort. When I uncheck 'enable backlash comp.' then the trouble is completely gone. Full speed any which way I like.
Mechanics, electronics, hardware, power supply, printer port, motors,accel, steps, duration, everything is proved good by this although I am still in the soft stuff as I need backlash comp.

The backlash speed % was 15% of full, I put it there after a quick run on the day I commissioned mach3. playing with this has shown 6%-8% makes things a lot better than they were. Though the x y motors are still doing something funny when pushed.

A bit of sniffing about, there are words like 'CV mode', 'shuttle acceleration' appearing. This is where I will look next.

Thanks for all the pointers
BFN