Heat Temperature Varies

My 2005 Honda Civic has just started doling out heat when it feels like it regardless of where I set the dial. It seem that when I am accelerating or applying gas, heat is available but when I am stopped at a light or coasting downhill, cold air is all that is available. Is this a thermostat issue or something more dire?

The first step is to check the level of coolant in the radiator (when it is cold).
More than likely, you are low on coolant.

However, that leads to the inevitable question of why the cooling system was low on coolant. If you need to add coolant, you also have to investigate where it is leaking from. Failure to do this could result in an overheated engine, and that will run into big bucks.