I'm having some fun. I have always worked in G90 mode, but the sopftware seems to want to switch back to g91 offset mode whenever it feels like it. Thankfully it hasn't felt like it when in teh middle of a GCode run - yet. Anyone got any ideas why this is happening?

Probably either some of your code or the initialisation changes it to G91.
Hood

Not my code, definitely- this happens when I'm using the shuttlepro to jog, mostly.  I'll set the zero points and jog off them then go enter a g1x0y0 in the MDI interface- and the thing doesn't move. So I enter it again, stil no movement. So I check to see what's wrong, and the 'Normal Condfition' flasher is on. I enter a g90 before the g1x0y0 and it works. I want to know why it's not holding a normal condition, because very soon after I enter that g90 I have to enter it again.

Well looks like the shuttle pro is changing it, I dont have a shuttle por so cant comment on it. One thing to check is that you dont have G91 set in General Config, it could be that the Shuttle Pro is resetting to the initialisation.
Hood