Hi,
I had same symptoms, would tickover in idle without over heating but blowing cold air from vents. As soon as started driving within a couple of miles it over heated. I was towing a caravan at the time. Turned out to be waterpump. The plastic impeller had come loose and was spinning on the metal shaft, apparently a common problem on pumps with plastic impellers. I know you've had problems with head gaskets before but wheras they cost money to diagnose properly, the waterpump impeller costs nothing and hence is worth checking first. After removing the thermostat put your finger in the hole and you can feel the impeller. If the blades on the impeller are broken or you can turn the impeller with your finger the waterpump needs replacing. When ordering make sure it has a metal impeller which will ensure you don't have this problem again. I didn't check on mine but if the pumps not pumping properly it should affect the amount of coolant returning to the reservoir.
If its not the pump then perhaps investigate head gasket.
