If the pressures are good I would check the temp door if it is working to direct the air flow to the evaporator core or the water valve in the heater hose to make sure its shutting the coolant off to the heater, whichever it has. I am not familiar with a 64 Pontiac as to which system it uses to stop heat from the heater core. Also on older vehicles that dont have a cabin air filter, I have seen crap like leaves or dirt plugging up the heater box restricting air flow in the heater box.
Paul

I don't have the numbers off hand for the pressures. A older fellow down the street checked the pressures (retired dodge mechanic) said pressures look goo but was not familiar with the system to guess what the issue may be. It does not appear to use a coolant flow shut off. There seems to be sufficient air flow but that not to say its coming from the right places. I have wondered if it could be debris on the outside f the evaporator and its forcing air over the heater core? But why would it be cold when driving and not when Idling? I wondered if the compressor is tired? Only 19K on the car. The evaporator is in the kick panel and I cant begin to imagine how you would get in to check it without a major dismantle.