Heat creep causing the filament to soften and then the extruder motor failing to push through reliably?

I had some poor filament that was causing the extruder hobbed gear to become clogged, which then led to the odd failed print.

Have you thoroughly cleaned the extruder drive? Use a craft knife to clean each spline fully, scrape any residue from the idler and lightly oil the bearing. Try changing the nozzle and again, scrape away any burned residue.

I doubt the stepper has gone bad and the difference in resistance is very small. A failing heater also sounds unlikely, as this would manifest itself in slow heating.

My experience is that most failures are mechanical, followed by cable faults. Even the bargain basement parts used by many manufacturers tend to last quite well and failures tend to be total rather than partial.