@spudaddict OK, so smoking gun found. I went to swap out the other stepper motor and noticed three case screws (on the bottom) were missing. I replace the screws and two stripped out and two were loose. I pulled the T20 driver gear off, and there is grease speckled underneath.
So, I am guessing without the case screws the internal bearings got skewed and that the motor is likely unrecoverable.
I tapped the motor case and got 2 screws to hold - luckily they are diagonally opposite each other. Just for grins I started my "tube test" design as 3 of the 4 tubes consistently turn out terrible. Here is what I got:
IMG_0946[1].JPG
Comparatively perfect minus some relatively minor tuning.
New stepper motor is on order and this is one to put in the troubleshooting books - if you ever get a repeatable bi-stable layer shift on a 45degree angle, check your stepper motor.
THANK YOU everyone for your input. I am implementing pretty much every suggestion I got as there are definitely other items that I can improve on. The improved belt alignment and stiffer front plane has been integrated into my new print head design. (and now I can even print it!!) Other improvements will continue. Thanks again - especially JamesM, mrehorstdmd, Danal who stuck it out to the end.
