Hi Graeme,
I found the problem with random Feed Rate Overrides. It was my code!
I was calling CSS (G96) along with Feed/Rev (G95). This appears to confuse Mach3. I deleted the G96 call and the problem went away.
However, the problem of spindle speed has always existed on both my mill and lathe. I have overcome this bug in Mach3 by issuing the following sequence:
M3 S500 (The spindle starts and settles at 640rpm)
G04 P2 (Stabilize RPM)
S500 (the spindle slows to the correct speed of 500rpm)
What I was hoping for was a solution to the spindle speed issue by substituting a "reset" command in lieu of the second S500 command. I guess that it is not that important as I already have a work around for it.
However, it would be nice to understand why Mach3 behaves like it does with initial spindle speed.
Regards
Chrisjh