Look for missing / lost steps on Bridgeport Z axis

Lost steps? WTF.
Yes, a week ago when I ran my first engraving since reassembling the Z axis on the Blidgeport, I found it was gradually shifting vertically as I ran the part. Didn't see this when cutting air, so I'm pretty certain it's due to noise from the VFD. Let's have a closer look, now I've got the Chinesium DRO working again.

Macro to cycle the Z axis:
I'm not about to manually run the quill up and down like some CNC wanker. Instead, a short g code file will do the trick:
N5   G54 
N10 G00
N15 Z0
N20 Z-100
N25 G04 P500
N30 GOTO N15

Or words to that effect. The G00 ensures a reasonably fast move, assuming that this would be a worst case feed situation. The G04 is the "pause" command. I need time to read the DRO and DTI when the quill is at full travel, being a human and all that. Then it loops back to the beginning endlessly until you hit reset.

Seems to work as intended:

With the VFD running but disabled, I'm not seeing any naughtiness. That is likely to happen when I start the spindle.

Oooooff!! Now the Chinesium DRO has started to talk bollocks. It's reporting 360mm drift. Sort of thinking that's not due to missing steps on the Z system. Sod it. Looks as if I'll be relying on the Mitutoyo (mechanical) DTI...

Turn it up!
Right, let's enable the VFD and get the electrical noise cranked up. I'm trying to recreate what I saw a week ago before I change anything, so I've not got any noise filtering slapped on yet.

Well, that's not doing much. So let's add 100mm G00 moves on X and Y, as well as 3000rpm on the spindle. Then cycle in a loop as before. I've moved the DTI to the head, as the table will be moving 100mm in each X and Y direction....

Hmmm. Not much going on here, even after almost an hour. Haha I suppose I could say I am almost going round and round in circles.

If this persists, I may be forced to re-run the original program. I'll have to decide if I should add noise filters first, as I have a life to live and plan to fit them anyway. That's not quite the ideal outcome, as it suggests I have a marginal condition here, rather like an intermittent fault. Not seeing the issue for now doesn't mean you've fixed it.