Overheating Problem "not waterpump" says dealership

I am having a problem with my 2000 Econoline overheating. I am posting this here because the econoline forum looks dead.

Problem
I am having a problem with the vehicle overheating. It seems to only happen when the air is on and is running for 30 minutes or more. Everytime I drive it for 10 minutes or more I smell that overheating smell when I get out. When It originally happened on the freeway all my coolant leaked out and I had to stop on the shoulder. The mechanic I took it to then replaced the leaky coolant cap and told me he was worried about the head gasket.

Dealer
I just bought this vehicle used and have taken it back to the dealer three times. They can't find the problem. I don't think they are looking very hard though because it would still be under warranty if they could find it. They won't even hook it up for a computer diagnostic because "it won't show anything" even after repeated requests.

Any advice with how to deal with my problem or the dealership would be more than appreciated. I am already weeks behind on a project because of this problem.

Thanks very much for reading.

Go to a good radiator shop and get it checked to determine if you have a head gasket leaking. They can run a gas analysis for you.
I would also go to another repair shop.

Rhandwar is 100% right. Take it to a radiator shop. That's all they do. Are you getting any bubbles in the coolant overflow tank? Any steam out the tailpipe? I'm thinking along the lines of new thermostat and getting the radiator rodded out. You certainly ought to get it fixed there in Motor City. They say, The Squeaky Wheel Gets The Grease. Be assertive with them.