Pontiac: Intermittant A/C, muggy conditions, high humidity

Question
I have a 2000 Grand Prix with the 3800 series V6 engine and about 65K miles.  About a week ago, as I was driving on the highway at night in very muggy conditions, I noticed that the A/C had stopped putting out cold dry air and cool, more humid air was coming out.  Then a short time later, the cold air came back.  It kept doing this.  I didn't notice it with non-highway driving until really the last couple days, especially today.  With the A/C on max, which should run the compressor almost constantly, it cycles between cold refigerated air and air that is simply blowing across formerly cold coils.  It seems to run better when I'm not driving the engine as hard (ie in-town streets vs highway). I had a mechanic test the system and he said the pressure checked, but w/o evacuating the system he couldn't tell what the mix was in the system, if it was mostly Freon or if it was an air/Freon mix.  While sitting in my drive, with the air running, I opened the hood and could hear the compressor cycling on and off when it should have just been running.  So my question is does this sound like a low Freon situation in which I should evacuate/recharge the system, or could it possibly be a low-pressure switch that's going bad?  Or maybe it's something totally else.  Thanks for the help!

Answer
It sounds to me like the evaporator care is icing up causing the compressor to shut down until it thaws. This usually only happens if the system is over charged but alot of humidity can cause it also. When you were driving down the highway you had more airflow through the condenser causing the system to be cooled down and then in turn, it causes the evap core to be colder. This along with the high humidity causes the icing to occur. Then when you are driving in town, there is more heat building up under the hood which is helping to keep things thawed out.
You could also have a plugged evaporator case drain.
Usually if you have a low refrigerant level, you will notice a temperature difference from the drivers side to the passenger side vents. Also the air will not blow cold and the compressor will cycle frequently.
Like I said before, this problem could be caused by the high humidity. I would wait a few days and see how it works when the weather changes.