When he mentions that a small piece of grit was fouling the gearbox and the pithy advice that small things can cause big problems reminded me of the toaster I fixed recently where I spent most of my time with the electrical aspects ruling out everything until I was left with the electromagnetic latch and after disassembly it turned out a very small toast crumb had found it's way inside and was in the way of a flush contact between the electromagnet and a steel plate. Just cleaning that out brought the toaster back to full function!