Chrysler Repair: No heat at all/ Gauge goes to H, head gasket leak, engine cooling system

Question
HI, I have a 1990 lebaron conv that just suddenly stop blowing ANY warm air,let alone hot. The temp. guage instantly within seconds shot up to "H" and all that came from the vents was ice cold air. I changed the thermostat and did get hot air for a few minutes, temp guage stayed in normal range. Unfortunately, that only lasted while the car was running. Later that same day, I went to run an errand and I'm back to square one...temp all the way up and ice cold air again; even after letting it run for about 20 min. or so. There is no wet floor on the passenger side or any antifreeze "smog" filling up the car, which is usually sure signs of a heater core problem. Totally lost at this point, any ideas???  PLEASE HELP!!!!

Answer
The race for "expert of the month" is going down to the wire.
Chrysler "repairer" vs Microsoft "coder".
One vote per answer is permitted.
Deadline to vote is Thursday, 4 PM CT.
Click on "Thank/rate" button below, then click on "Yes" for nomination.
Thanks

Hi Scott,
With a temp gauge shooting up to H I would suspect that you are loosing coolant into the compression chambers due to a head gasket leak. Check for whether the oil is foamy or whether you get execessive white smoke out the tail pipe when you first start the engine from cold. If so, that would be symptomatic of such a leak. Also, if you hear bubbling early after you start, coming from the engine, that is another sign or early pressurization of the radiator(remove radiator cap after a minute or so of idling carefully). It should not be pressurized that quickly. Have you also run out of coolant, perhaps?
Until you get the engine cooling system up to snuff you won't get any water circulating to the heater core. Let me know what you learn and also tell me which engine you have.
Roland
PS:May I ask a favor of you? I am seeking the recognition of being the 'volunteer of the month' at Allexperts. To do that I need your nomination which you can give by clicking on the "Thank/rate the expert" button below. That will take you to a page where you will see the question "Would you like to nominate the expert to be volunteer of the month?" If you would click on "yes" I would be most appreciative. Thank you and Merry Christmas!