rough traversing and stalling only in main screen

[davethefixer]

Hi all
i have a odd problem with my latest build.
i am having problems with traversing smoothly.
my system is as follows
X axis -two RM2010 ballscrews 1500mm long, belt driven (one to one ratio 40 tooth HDT pulleys)
from single 8.7nm nema34 stepper, hiwin 20mm profile rails, CW8060 stepper drivers, 50v 600w power supply.
now from the motorcalc spreadsheet my max rapid is 3800mm/min
i have set up all the various parameters pins/ports ect.
now thw crunch.......
in the motor tuning screen steps are set to 40 steps/unit
at 3500mm/min the carrage moves smoothly with little noise PERFECT........... BUT........
as soon as you save and go back to the main screen and try to traverse it stalls out and is very jerky with a lot of noise from the stepper motor.
it does this on all axis traversing from the keyboard or running gcode.
go back into motor tuning and its smooth as smooth again.!!!!!!!!!!!!
iv been through the Mach3 manual searched the forums but have not found anyone else having ths problem.
im hoping someone has had this problem and knows how to solve it.
heres a link to a shakey video i took(mobile phone) problem cnc - YouTube
heres a few photos (i will get round to a build log soon)


Thanks all