No Heat and its cold

I replaced my thermostat today, but I still do not have heat while idling. I can go down the road and it will give off good heat and then go cold and back to hot. What the heck is going on?

There might be air trapped in the cooling system.

To remove any air from the cooling system, get the engine up to operating temperature. With the engine idling, slightly lossen the upper radiator hose clamp. Take a flat bladed screwdriver and slip this between the upper radiator hose and the hose neck on the radiator. Allow the engine to idle until all that comes out of the upper radiator hose is coolant. Retighten the hose clamp.