Stepper motors don't run smoothly

In a recent topic I asked the question why my X-Carve spindle would judder when travelling in the -X and -Y direction. It was suggested that the X Maker Slide needed the stiffening mod so I carried this out and yes it did help and thanks for the pointer because that was something that needed doing.
I also suspected a worn out spindle so I’ve changed that too. Thinking that I’ve now got the problem beaten I started my 430mm circle cut in a sheet of 12mm MDF (just a test job).
To my horror it’s still shaking! I have just done what I should have done at the start of diagnosing the problem and that’s to set a job going that only cuts air. What I’ve discovered is that the shuddering happens all the time irrespective of what is being cut (nothing in the case of this test).
From what I observe it seems that the stepper motors are juddering. I’ve tweaked the voltage to them the other week and I assume I’ve got that right but who knows.
Any suggestions please?

Check your electrical connections, especially along the back terminal block and the side block where all the motors connect up. Make sure you don’t have any loose wires or screws.

When the motors judder, do they lose steps? If you’re trying to cut a circle, do you get an oval or an odd square-ish blob thing? Or do you get a circle (or several) in a different location?

All connections seem fine.
The cutting is fine and no steps are being lost. Circles come out perfect circles.
I thought it might be the wheels so I’ve checked them to make sure they’re not too tight or too loose. Could it be the belts I wonder? They give a low pitch twang when I twang them.
It just seems odd that I get juddering in ±X and ±Y