2003 F150 run hot/ blows cold

A friend has a 2003 F150 that after starting up will run hot but blow cold air when the heater is on. The truck will run hot for ~ 5seconds then all of a sudden the temp drops to normal range and the heater begins to blow hot. Simultaneously, the oil pressure light comes on when the truck runs hot, then falls to the normal range when the temp drops. I thought perhaps the thermostat is in backward. any other thoughts?

Yes, the temperature spiking could be caused by an improperly installed thermostat. And overheating causes coolant to boil out which air locks the heater. Check/replace the thermostat and burp the heater hoses.