Extrusion failing directionally?  

Extrusion failing directionally?

I am having a problem where it seems I am getting bad extrusion, but only in certain directions.

So I have just put on a new roll of filament (Prusament PET-G), and attempting to print using the same settings as before (with the previous roll, also Prusament PET-G) was failing terribly.  I was ending up with a sticky blob of filament on my nozzle.  I had also made some minor modifications to the machine (mounting the power supply to the table, as per the Lack enclosure instructions), but nothing significant.

I decided to re-calibrate X, Y and Z.  And my z-level, using the same steel sheet, is now a significantly different value to what it was before (1.625 -> 1.675 or so).  However, I believe the z-level calibration is now correct (here is the sample square printed at 1.675 for the whole thing):

However, I can't get a print to start successfully.  When the bed is moving forwards (so positive Y rate), with a slight negative X rate, the extrusion fades, and blobs.  Once it was more negative X than positive Y, the extrusion line looks perfect again.  However, once it finished the skirt (which you can see, is not complete), and tried to start printing the base for a support, it clumped up and was a hot mess.

Is it possible this is nozzle damage (a notch in one direction)?  Or a defect in the new roll of Prusament PET-G?  I've done this on a completely clean bed, washed with soap/water and thoroughly dried.  I get the same result.

I might well be missing something, but I'm not sure what.

Preston