2006 Frontier has only intermintent heat. Anti freeze levels good, no antifreeze leaking or in the crankcase. Normal temps on the gauge.The heat just comes and goes

Be verry careful with this kind of problem. Make sure you are not losing coolant somewere like through the exhaust through the head gasket. Before hot but close to opperating temp check for air bubles while radiator cap off. IF there are bubbles then the head gasket is leaking or blown. Sometimes a bottle of head gasket sealer will do the trick. (Remember if there is no fluid the electric thermometer for the engine to read it will not say it is hot). If the temp rises at a stop and then decreases this may be the problem. Check the fan clutch, if it feels realy easy to turn by hand then it may need to be replaced. Coolant systems (not open systems like BMW) must maintain constant pressure, so if the seal on the radiator is bad and air can escape then it needs to be replaced.