Gregan,
Mach3 has a problem with tangential control - when it comes to cutting circles the blade will, under certain circumstances, rotate whilst it is in the work. The only satisfactory solution I have found to this problem with G2/G3 commands is within the CAD/CAM program (Aspire) to create lead-in's and lead-out's with exactly the same radius as the circle being cut. Blade rotation then happens during the lead-in / out and not within the work.
I am not certain if the problem exists with arc-tangent curves as I have always used G2/G3.
Hope this helps.
Tweakie.