Could a step-down converter cause this issue? I have the same issue as OP and i'm in Europe using a 240v-110v converter to power a Nanao MS9-29 monitor.
It guess it might be possible.
But OP's issue doesn't happen with PCBs, so my guess is the issue lays on the PC, rather than the cabinet.
My debugging "algorithm" would be making 100% sure it's not a timing issue, by testing a 57 Hz sample mode in Arcade OSD and playing with all relevant geometry settings (horizontal). If that doesn't help, then try doing changes on the pc hardware, try with a spare psu, a different card, etc. If the cabinet has fluorescent lamps, try to turn them off, etc.
I currently have a similar issue with one of my cabinets, it started happening after a house move. But here it doesn't depend on the refresh, it happens with all modes, and it gets better for I while when I press the degauss button, so I suspect it has to do with the degauss circuit. It's hard to notice unless you look very close.
What you need to understand is this IS a hardware issue. Whether or not this can indirectly be fixed by software means is a different thing. What you're seeing is a horizontal sync pulse which length or position is not stable.