Radioactive, I don't know anything about this specific fixture offset problem, but there are unfortunately other problems with fixture offsets in Mach3.
If you do a feed hold and then start in G code with fixture offsets you're asking for a big crash (and this got me pretty bad), Mach will often apply the wrong fixture offset when the Start is hit.
A workaround for this problem is to put a dwell operation right before the fixture change, ie:
G4 P1 (Fixes Mach3 Feedhold problem)
G59 P#101 (Change to Specified fixture offset)
I don't know if this workaround will help with your problem, but its worth a try, let us know if it helps.
I really hope this gets fixed in the new release, as far as I'm concerned this is "Class A" critical bug that shouldn't be present in a Mach release version, a machine or a finger can really get hurt with problems like this.
Paul T.