I almost convinced myself it was thermal. Vref was at 1.6V - over the "1.5 max / 1.3 max recommended", so I was running in the red zone. Backed to Vref=1.3V, works fine for now. The motors are dead cold, but there is almost no torque required to move that Z carriage anyway. I was not solving any real problem by increasing that Vref.
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