XYZ Calibration Problem  

XYZ Calibration Problem

I've had my Mk3 for several weeks now, and in general it has worked pretty good. Definitely things to tune up, but I am working on that.
My last couple prints had a couple dramatic X-axis shifts, which I have seen before. It seemed to be corrected last time by tightening the X-axis belt, so I gave it another go.

Self-test passed the X-axis, but then failed on the Y. So I tightened the belt there too, and selftest worked like a charm.

So I ran the XYZ calibration, and it failed.
The 1st pass seems to work just fine, but on the 2nd pass (Improving Calibration), it does not seem to actually be hitting the calibration point on the 1st point - it is "scanning" back and to the right of the circle, so that only the most extreme left of the X-axis and forward of the bed will hit the spot, It churns away on point 1 for what seems like forever, and eventually goes to point 2 which is dead on. Same with points 3 & 4, after which I get the "calibration failed, check the manual" message.

It seems that I should try adjusting the PINDA height, which I have tried.

Seriously, many, many times. with the same result.
I've tried factory resets before the cal, running the wizard, running the XYZ cal alone, running the selftest then the cal, but nothing seems to work. I don't know how many times I have tried the various combinations of these things... nothing seems to work!

This is with Firmware 3.1.3, which worked just fine before, after my last belt tightening.

Please, does anyone have any advice as to what I can try next???

Thanks in advance!!