Sudden loss of all electrical power while idling - everything shuts down - engine off, power steering goes, radio turns off, clock needs to be reprogrammed, etc - been told its not the alternator or the battery - what else could it be?

Hello, sounds like a loss of battery voltage. Check all battery connections, check to ensure all cables are free of corrosion. Because it seems to fix itself, I would focus on vehicle wiring and connections. Hands on testing and movement of wire harnesses would be a good step to take during running of the vehicle to see if the stall can be recreated. You could have other electrical component issues, but until you can test while the problem occurs it will be difficult to isolate.