[LMP]

Simple solutions first: check coolant level, check for kinked hoses, tightly capped radiator......
NOw...randomly heating/non heating heater can be caused by exhaust intrusion in the cooling (heating) system from blown head gasket...but this comes with other symptoms, like steaming, overheating of engine when you freeze inside the car..haha..no, not funny...but I just wish these symptoms do not prove themselves.