Y-axis stalls when cutting circles but not when cutting squares. I'm using Vectric Aspire software. Vectric Aspire kind of blames Mach3 for the problem. Anything to that?
Try setting lookahead to 200 lines or even 500 lines (default of mach3 is 20 but default of Mach4 is 200) and turn on CV mode.
If you are NOT on demo copies of Mach3 (limited to 25,000Hz), the you can set the kernel speed up to 36000 or 45000 or 60,000 depending on your PC.
My Win7 32PC with Core I5 7600 can have kennel running at 100,000Hz with excellent result. So I increase my microsteps from 8 to 16 to get smoother curves.
Aspire postprocessor probably is now using many small straight lines instead of arcs to cut arc circles. You may need a better post processor (or settings) for MACH3 in Aspire.