Ok, I am intermitttently having this problem. When starting I will occassionally get this shitty idle like a vac hose is off that will usually just go away inside a minute but at times it also will hang, inside 2 mins the car is overheated and the interior heat blows cold. When not having the idle/overheat problem, the heater works fine. If I rev the engine a few times the idle will usually normalize but hwen it doesn't it is guaranteed to overheat. I have checked when this happens and the thermostat is not opening. I replaced the thermostat 2 weeks ago, have flushed and filled the cooling system, radiator is fine, water pump is working, when this set of symptoms do not occur, I can drive however far/long I need (10 mins or hours) without it overheating.

Could this be the FITV or am I looking at another problem here?