Just to add to the list of things that could drive you balmy, you may also wish to check the heater control valve. On the 6 cylinder cars this is located against the firewall in the engine compartment, on the driver's side. Access is absolutely horrible, perhaps better done from underneath if replacement is called for. You will, however, be able to reach underneath it from above with your left hand, so see if it's leaking from cracks or worn hoses. These leaks can otherwise be nearly imperceptible (ask my how I know). If the valve has failed internally that might explain getting cool air even when the heater is set all the way to hot.