Hi,
You are correct, this is unusual.The first thing that comes to mind is one you are not gonna want to hear, but due to the erratic nature of this beast, I must mention it, anyway. Are you CERTAIN you have a full load of coolant? Low coolant does nutty things to a motor. If that is ok,then I would again look at the waterpump or thermostat.When filling these cars, put the heater on, so the control valve is open.Also, make sure you have the correct waterpump, as I think these impellers are backwards in their rotating direction. Last, but by no means least, a top quality thermostat is vitally important,installed correctly. Given your symptoms, there should be other problems evident, besides just no heat once in a while, etc. Let us know.