1999 Honda Accord Temperature

Is your new thermostat the same temperature rating as your old one? Is the heat from your heater any different since you changed the thermostat?

I'm not saying there is a problem with it. If there is a problem with the heater blowing cold air then-yes- there is a problem. The t-stat will allow the coolant temperature inside of the engine to get to its' rating. Then it opens and lets the coolant flow through the radiator, where heat is extracted from the coolant. When the coolant is cooled to the t-stats' rating the t-stat closes and starts the process all over. Your heat gauge will fluctuate as this process repeats itself over and over. This fluctuation is not a problem.

A leaking head gasket will cause coolant to gush out and the engine will sputter, spit and misfire. You say the engine runs great. Turning the heater on and off will not cause a steaming head gasket leak. A failing head gasket will leak all the time once it gets to the point of steaming. When you say you turn off the heater do you mean the fan or the interior temperature switch?
Before you run the car again, check the coolant level. Make sure it's full - my guess is that it isn't. Add a little water if it's not too low. Now run the engine and get it to steam again. Open the hood and see where it is coming from. Coolant is leaking somewhere, check around the where the thermostat was replaced. Check especialy around where the upper and lower radiator hoses connect to the engine. Let me know your findings

Good advise. Don't forget the radiator. There can be a crack in the radiator that releases steam and even worse does not allow the cooling system to pressurize. Nip this thing now or you will be having head and head gasket issues.

Ah yes, good catch Bruce, I've seen these radiators have a hairline crank in them. Especialy in the plastic that holds the fins together. Thanks Bruce