Hi Andy - you switch between the 2 modes by doing a click (press the mouse button and then release it without dragging) on a corner grip instead of holding down the mouse button and dragging on a corner grip.

When you just move the mouse button over the grip it will show a little origin point graphic to show you which mode is currently active (either "scale from center", or "scale from opposite corner").

If you're getting different behavior at different times, it means that you've done some clicks instead of drags on those grips at some point in time there.

Also if you click on the rotation grip it will go into "wheel mode" where you'll be able to grab the center point and move the center of rotation to some particular spot.

For more info see here:
http://moi3d.com/2.0/docs/moi_command_reference11.htm#editframe

- Michael