Triac home position error (y axis)

Hi all,
I’ve been having an issue with my Triac that I just can’t fathom!
At first I thought it was programming error on my part, but I’m quite sure I’m not to blame after all!!

I’m getting an intermittent issue that only seems to effect the y-axis.
Machine homes in XYZ with no issue.
After a G28/home/Toolchange *occasionally* when returning to position, I would get a deviation of approx. 5mm to the negative; i.e. tool at X0Y0, toolchange, return to X0Y0 by shown program position but physically returns to X0Y-5.0.
My datum stays constant so program position and machine position are shown correctly, but are physically out.
Re-homing out of program resets to correct position and X0Y0 is actually X0Y0.

I removed bellows and cleared swarf and chips from the casting and around the switch gear.
Cleaned it all, contact-cleaner applied to the micro switches and tested fine.
Positioning, toolchanging and repositioned fine, though didn’t get to ‘run in anger’.
Went to make a part tonight and had the same, but the Yaxis deviation was more like 10mm!!

Program is all centreline, with no comp. But I am running a 5mm slot drill.
I can’t see how it would be a factor, but for ‘full disclosure’.
(Though it did cross my mind…more than once!)

What and where do I start with getting to the bottom of this?!
Your help and advice is appreciated!!