2012 Jetta SEL overheats, red temp light flashing, A/C stops cooling when this happens, temp light just flashes never solid on. I have already changed the water pump, coolant tank, temp sensor. No change! Coolant tank level goes up and down, not getting much circulation from the return hose, just spits coolant and mostly hot air! Only thing left to change is the thermostat? What is my problem?