Hi there, I'm new to this forum. I have the 4 litre SC400. Now it is getting hot. Almost like the thermostat is sticking. It gets really hot and the heater blows cool air. Then , as if an air bubble just yet loose, the heater will bow warm air and the temp will come down. Not all the way, but down to a safer level. Do I have to pull that bung on top of the thermostat to bleed an air lock?