About a month ago I was driving my son to school when I noticed that the blower was blowing cold and the temp guage was rising towards hot. Almost as quickly the needle went down and the heater started blowing warm air as normal. I thought no more of it until two weeks ago when it did the same again, this time getting into the red (I couldnt stop as soon as I would have chosen to as was on remote windy road but the engine didnt appear to boil up). I wondered if the previous owner who had supposedly had the cam belt and water pump changed had checked to see that antifreeze had been added as it was very very cold that morning. I let the van cool and returned with 5 litres of antifreeze but the cooling system appeared full. Nevertheless I drained some water and refilled with the antifreeze and the problem got better but the temp guage still 'wandered' more than normal and every now and then the blower would blow cold. I changed the thermostat but still, normally when driving first thing in the morning, the guage wanders and I dont always get warm air consistently until about 30 mins of driving. I wonder if there is an airlock in the cooling system?

Any help and advice would be much appreciated! Thanks