Hi Mike,
You mentioned in post #5 that your outside diameter was not completely round (my out side dia. is supposed to be 12 1/2" well in Y axis its 12 1/2 but in X its 12 3/8"). Checking your Gcode indicates that Mach3 will be following a toolpath which will produce equal dimension in both the X and Y planes and this is confirmed by the Program Limits of +/- 6.3438 (shown in the toolpath screen).
It would appear that you have a mechanical issue with your machine (backlash or flex, etc) which is causing the inaccuracy in the machined dimensions.
Perhaps ensure that the cutter you are using is truly 'sharp' and possibly reduce the feed-rate or depth of cut to reduce the loading on your machine components.
Hope this helps,
Tweakie.