I have a 3.0 truck, the last couple times I've driven the truck the heat seems to go off and on without any reason. The heat is always set to max and hasn't been moved. The truck will blow great heat, then for no reason blow cold air for 10n minutes, then back to heat. I'm kinda confused.

Coolant level is good.
Heater core is new.
Rad is new.

Anyone else experience this?