Ran the program again and I believe the error actually arrives at line 47866: X-58.484 Y30.804

For some reason the Y moves fine on that move, but the X stays put, the stepper making the weird noise like it is hitting its limits. Throws the X coords off by 58.484mm.

I think that something in the preceding 1000 or so lines of code creates this error: I couldn't replicate the error by doing a "Run from here" from ~47000 but could replicate it when I ran from ~46000.

Thanks CitizenOfDream; I get your logic and I'd give it a try, but the same exact error happened yesterday when I was running it with much lower feedrates. I'm not sure what CNC controller it is to be honest, it is a Denford Novamill that was converted to Mach3 by someone, not sure what they used.