Thanks Tweaky,
It ended up being total user error. I had recently changed the post processor that snuck the G64 into the invocation section that you mentioned. It being modal caused the same issues with g-code files that I didn't have issues with before, which is why I was puzzled.
Anyway, I removed the G64 reference in my postprocessor so the Mach setting is in control, and all is well.
Thanks for leading me down the right path.
Lynn