Tony,
I'd suggest downloading version .38 from here:
ftp://machsupport.com/Mach/I'm having loads of trouble with many things in version .40 using parallel port, "std. PC" and Win 2000. Actually I've totally given up on version .40.
I did experience what you have with .40. I found only way to get it working again was to stop Mach and open it up again. Also, it would get stuck in other jog modes also. And if you feed hold, often jog would get stuck when you least want it to.
Also, when you re-start from freehold, if it does manage to let you jog, the restart moves to get back to the path would be totally wrong. Use with great care! Not loosing any steps though.
Also, I get no spindle speed (with started spindle) using vn.40 and nothing I can adjust will get it back. Works fine with vn 38 and older.
But, there don't seem to be many other complaints out there for this version so there must be something with our set ups.
Vn. .38 seems generally better but has feed rate and re-start problems in Turn that I'm just trying to understand now.
Give it a try. I'd be very interested to know if .38 is better for you as well. There was a change to the driver in vn. .39 according to the log.
For me Vn 22 is good for Turn (not for threading) and vn.38 is mostly good for Mill but definitely not for Turn.
Woody.