Foghorn,
If you run any two pairs of speakers together with your switcher, the combined 8-ohm impedances will present a 4-ohm load (or perhaps even lower, if there are impedance dips on one of the speaker pairs) and Yamahas do not like 4-ohm loads. I think it's a combination of poor cooling and the low impedances when you run more than one zone. The thermal sensor would definitely shut down the Yamaha to prevent overheating, and I suspect the Yamaha also has a current-sensing circuit with current-limiting provision. So when it senses more current flowing to the lower impedance, it limits the Yamaha's output or shuts it down.
Domestic A/V receivers are not designed to drive multiple speaker sets in this sort of application. The advice on proper ventilation is sound. It might make a difference.
Regards
