I have a 2007 Ford Five Hundred and it has a very odd heat problem. When I first start it it has heat, no matter what temperature I have it set on. But after driving it a while the only way I can get it to give me heat is if I set the temperature control as high as it will go. What would cause it to have heat at first, then stop and possibly even odder- not have any heat unless the temperature control is on the hottest setting.