Had a bit of a hangup.  My nice gears suddenly started to turn out like this.  

Look close, and you can see the gear profile is screwy with bits sticking out, like the entire thing shifted (like an axis was selectively losing steps).  However, that wasn't it...

I eventually traced this down to one of the heatsinks on an extruder stepstick not being 'on' the chip.  I run them at basically full current so... that matters.  So what I think was happening is that one would overheat and then extrude *more* for some reason.  Strange.

But, I didn't figure that part out before I replaced my existing DRV8825 water cooled axis stepsticks.  

I replaced that bit of 'engineering' with some, now air cooled, AMIS-30543 modules and an Arduino to handle setup/enable/disable via SPI.  The DRV8825's have worked well here actually, running at their maximum current with essentially unlimited cooling, for the last 2 years or so

But, I had been meaning to try out the AMIS drivers for a while, so this was as good excuse as any I guess.  This actually pretty much sums up 'observer bias'.  Also, this is why you probably shouldn't tell the auto repair guy what you think is wrong with your car, even if you think you know.  :)

In trying to implement the AMIS interface, I actually ran out of pins on my Beaglebone Black to be able to do more than a single channel of SPI.  Kind of impressive, if I do say so myself... :)

The new drivers have a little bit higher current capability but I'm not using that yet. In any case I'm back up and running and printing.

<EDIT> Print finished, and the printer is back to being glorious.  Printing PLA with zero retraction (because I can do 480mm/sec rapids, and that works just as well and is much faster), and printing at 90mm/sec (minus outer walls), and the prints were perfect again.  Pictures, in a more on-topic log entry.</EDIT>