Sorry, took me a while as I was travelling.
The IJ and distance mode are the same as yours.
I let my cam sw create the nc file also in "convert to points" method, to have the option to mill w and w/o G2/3s.
I also went to the newest version of MACH.
My x axis speed is set to 750mm/min. When I use the arrow buttons on my keyboard the axis moves at 750, sometimes a little (745+-) slower.
I can drive it manually all over the place with no issues. Y axis is set to 400 or so, no issues here too.
Now I am loading the file I posted (with g3 and 450mm feed rate). It happens always at the same command: X-axis DRO is counting, axis does not move. When I touch the motors, I can feel IT SEEMS TO GET PULSES !!
Now I hit ESC. Machine stops. I reset emergency and can (arrow-keys) move x with 750, y with 400, no problem.
I even can, and that's crazy, take another file which mills a circle (G's !!!" at the same machine position at the same speed. No problem.
Now I take the "no G2/3"-file. Program runs...machine gets stuck close to the same position where it hangs up with the G3 file.
When I am forcing the feedrate down to 160mm...no problem....except time :-(
Processor is a 1.6GHz ATOM, XP with absolutely anyting else turned off.