There can only be one of a couple of things that will make it so you can not get round circles.
1) Backlash can be set to high or too low. Mach3 has backlash settings you can adjust easily. Try adjustments with a pen instead of wasting money on material.
2) You may have CV (ON) at the same time as the backlash enabled. Use one or the other but not both at the same time.
3) Something is lose. A lose key on a pinion gear, a lose pinion gear to the rack gear, a lose belt, anything lose can do exactly what you are explaining.
I had the same problem on my DynaCNC table and a couple of adjustments and tightening up of a couple of items and I get perfect circles every time. Check all of your alignments, leveling, and gears. If everything is tight and true, then try adjusting the backlash. Any mechanical machine has thing come lose and needs adjusting once in a while.