X,Y axis: 40,000 pulses/in, max 37.5 ipm, acc 5
A-axis: 2000 pulses/"in" (rotation), max 750 "ipm" (rpm), acc 200 (this acc value was larger than it should be, I miscalculated, but the intertial load on the axis is fairly trivial anyways, like an oz of foam 3" in dia)
location began as X1.88 Y1.4065 Z0 A0
G1 Y1.065 A2 F3000 caused the Y-axis to stall.
F3000 was put in as a "just the Mach3 limit on the A-axis rate decide". You kinda have to do this, because AFAIK otherwise if you enter F750 Mach3 calculates speed as (X^2+Y^2+Z^2+A^2)^0.5, right? Well in truth it wouldn't slow down the A-axis much seeing as the X & Y units are much smaller, but it's still counterproductive. In any case, all the code has this same Feedrate and it causes no problem.
Also I'd readily accept that it was a problem with the A-axis acceleration, like I say I know I made it too large, even though it didn't happen elsewhere in the code- except for the simple problem that it wasn't the A-axis which stalled but the Y.
Which, no, I can't make sense of this either. Which is why I was hoping there was some sort of debugging method showing some kind of level between its interpretation of what the G-code was asking it to do and the low-level pulses. But I haven't seen any indication that such a thing exists.
Of course having discovered that the A-axis problem I can set this back up and retry it again. I dunno, I've made some changes since then, this may not even be reproducible at this point.