Low coolant due to a head gasket leak or compression pressure blowing the coolant out through the coolant surge tank overflow can also cause big swings in coolant temperature.

The situation mentioned in the opening post in this thread doesn't indicate low coolant, but we're hearing of more incidents like this as these trucks age.

So, I would check the coolant level whenever there's a big swing in engine coolant temperature (usually felt in the heater output).