You have a small head gasket leak. Pressure in the radiator bleeds down each night into the combustion chamber. Have an air pocket in the water sytem at the top. When you startup in the morning, after a few minutes, the thermostat opens and soom the temp probe is uncovered and spikes way up. Drops back to normall in a few seconds. When you have no heat is because there is air inside the heater core, which sets up high. You probably have to add 2-3 cups of antifreeze after that , right ? I had the same thing. I use a Bars Leak head gasket and block sealer in the antifreeze and that fixes it. Have had the problem for 3 years. I have to retreat it about once per year. Originally i used the Bars Copper block seal for $10. This last time i switched to the more expensive 1111 for $30. to see if it will last longer as they claim.