A thread not long ago was titled something like "Mach does not honor accelleration [in blah blah situation]"
I'm going to throw out some very sketchy info here, but it is all I know at this point. I just tripped over this yesterday and have not had a chance to look up the thread on it, but since the beahvior you describe is exactly what I was getting, there may be a connection.
I'm still looking into this, but basically, it seems that under certain conditions, which I think *may* be related to CV being active or not, Mach will not honor the accell setting and instantly moves a stepper from zero to commanded speed.
The result of this was immediate stalling of my servo motor whenever the stepper slammed into motion.
As in your case, lowering the axis speed setting to stupidly low levels cured the probem, but I found a change in the program also remedied the issue, indicating it is likely a Mach bug.
In my case I had the A axis rotating fast (servo motor) and the X axis (stepper) stationary. At a point, I began the X and A moving together and WHAM. Very obviously the accell setting on the stepper was ingored by Mach.
Changing the program to eliminate the initial A only rotation completely eliminated the problem. The command G1 X.5 A1800 that was causing the stalling was unchanged. I simply had eliminated the immediately preceeding G1 A360
Things you might look for:
Is there an extremely sudden movement (or attempt at same) from the attempted motion causing the stall.
Turn off CV and see if there is any effect.
If you can edit the G-code, you might try adding another axis move to the Z move.
-or-
If the Z move is G0, change it to G1.
I'll try to post back as I learn more about this bug.