I have an 01 Ford Explorer Sport Trac. The past few years I have had to add freon to my vehicle before each summer in order for the AC to work. However this year, it only works when I'm driving. I checked the gauge that comes with the can of freon to see if it was charged, and it was (between 35-45 psi). The compressor clicks every 2 or 3 seconds, and only blows warm air. I have asked people at automotive stores what could be wrong, and they all said it sounds like it's low on freon. When it's undercharged, the compressor doesn't kick on. But when it's between 35-45 psi, it engages and disengages too often. Also, the radiator and dryer have been replaced within that past couple of years. Any ideas?