Replace the radiator cap. Be absolutely sure you get the correct one.

Unless I am missing something here, from your description, if you always get heat on the passenger side, that means the heater core is still hot and the cooling system is not air-bound. If it was air bound, then there would be no flow through the heater core and no heat on either side. I think you may have a problem with the flaps in the air ducts that divert the hot air stream to the driver'* side. There are some vacuum cans under the dash and a vacuum reservoir on the engine side of the firewall that you will want to check for disconnected or leaking hoses.

I am not sure exactly how all the flaps are arranged, but your description of blocking the dash vent to give heat on the other side implies that a flap that should be open is not.

The heatercore seems to be working fine - when there'* no heat the back hose is cold - for some reason the antifreeze isn't circulating through the system when I lose heat but when we blow the system everything flows through fine so it isn't showing any blockage anywhere. We've pulled the dash apart inside and outside and all flaps open and close like they should. It'* got a new radiator cap, thermostat, waterpump and hoses. This is getting really old as the temp. here this morning was 3 degrees - not an enjoyable ride to work this morning - I eventually had luke warm heat on the drivers side -the reason I say it appears to be airbound is that when we open the bleeder the fluid doesn't pour out like it should and then it starts to spit out and then will start pouring out so that'* why they're saying it appears to be airbound.

HELP!

I forgot to say - no, there isn't climate control.

OK, maybe the way the ducting is set up, when the heater core is starved, the driver'* side gets cold first. But with no flow, the passenger'* side should not stay warm very long. That cold heater hose is a definite indicator of no flow and the bleeder situation is a good call for air bound. So the question is: how is the air getting into the system (if it is air!) Or...how is coolant getting out of the system?

Do you see your coolant level dropping? [edit: you need to check the radiator coolant level. If you have a leaking system, the recovery tank level can stay up while the radiator level falls.] Are there any indications of an oil scum or dirty ring around the liquid line in the recovery tank? (leaking head gasket)

Have you done a pressure test on the cooling system? This might reveal if you have a bad o-ring somewhere or a pinhole leak that only opens under system pressure. It could also be your upper intake just starting to fail around the EGR stovepipe. If a pressure test goes OK, have someone with an engine analyzer stick the probe in your coolant recovery tank to see if the problem isn't air but exhaust gas, indicating a leaking head gasket.

Internal leak possibilities: cracked or leaking intake manifold (upper or lower). Cracked block or leaking head gasket.

None of this is good news, but, if it is the upper intake, or worse, you have caught it early. And, it hasn't failed catastrophically in the middle of the night in the middle of nowhere.

Coolant levels are right on at all times. Did the pressure check and it'* holding fine. No indication of any leaks from what we can find. The passenger side always blows nice and hot (that'* the side we have blocked off with small holes poked in the cardboard so there will still be some heat blowing) after we had blocked it last week I had hot air blowing from both sides (it was wonderful!) then on Saturday the weather was "nice" and I turned the thermostat down a little into the cooler air and that'* when I lost the heat again. We then bled the system again and checked the vents to make sure they were opening and closing properly - which they are - and I'm back to luke warm air on the driver'* side once in a while.

When you bleed the system, are you consistently getting air (or gas) out of it?

Consistently getting air - when you open the bleeder it spits and spurts out then nothing and then it'll pour out - it takes many times to get it to pour out consistent and like I said - it doesn't last. That'* why we're saying it'* airbound.

Theres a block tester that checks for exhaust gas in the radiator. Its the only
sure way to tell if a head gasket is leaking pressurizing the sytem with air.
My Supra is doing a similar thing where it overflows the coolant bottle and has to be purged of air in the system.
You might also want to flow check the radiator and heater core to check for blockages.

To me sounds like some sort of vaccum leak within the firewall or under the hood. Even could be the damper assembly itself, had a few go in mustangs before... When my dampner switch go, i would get driver side vent only, or vent and windshield, but it never worked 100%... Sounds quite similar...