Raspberry Pi 4 with Mesa 7c81

Absolutely. Will try it shortly.

I spent a lot of time reading about ferror last night and don't completely understand how that works on an open loop system...

Mesa (and I think Pico systems) stepgens are actually run in closed loop mode
though the loop is closed in hal. Basically the step generators run in velocity mode
and the local PID loop monitors the actual step count (and fractional step count)
and adjusts for minor deviations caused by differences in LinuCNC and the stepgens
clocks, lack of frequency setting resolution etc etc.

It would appear that changing the ferror and min ferror values has solved the problem. I will run a few more tests to be confident. Much thanks again to PCW and andypugh! You guys are awesome!

Also thanks for that explanation of the ferror. Makes total sense now that you said it's closed in hal. That was the part I was missing!

Changing the ferror limit will have no effect on the motion _except_
it will trigger a following error and fault if the hardware stepgen gets
out of sync with the commanded position by greater than the limits.
I suggested reducing them to guarantee that the stepgen is doing what
its told (if not it would fault and stop motion)

After reading this I get the impression that this shouldn't have fixed it? Is this only for and error message generator or does this actually control the maximum allowable error?