Blowing Hot Air Through the Dash Vents

My 2002 recently began blowing hot air through the dash vents, as if the heat were turn halfway up. This was more than just the initial blast of hot air you get after the car has been sitting in the sun. This keeps up for miles. But if I turn the car off and stop for a while, the problem often corrects itself. I took it back under warranty to Koons Ford in Baltimore and they told me this was not a problem but was an "operational characteristic" of the Focus - a clever way Ford devised to keep the engine from overheating on a hot day. Now, I'm in all in favor of protecting the engine, but not at the expense of the passengers. If I turn the Air on, the problem seems to go away, but this happens on days I would not need air conditioning, were it not for the hot air being pumped in from the vents. Has anyone else experienced this "operational characteristic" or did my dealer just not feel up to looking for the problem? I have another Focus that doesn't have this problem, nor did this one until about a week ago.