I have a 99 with 5.2 and the air conditioning blows nice and cold for about 4 to 5 minutes then eventually blows considerablywarmer air than what is outside. When the temperatures are in the 80's with low humidity it works fine but when the temps are in the 90's and high humidity it does this. If I wait 5 min. or so it will blow cold for a short time again then warm. I know the system is filled properly with the gauge I have so I know it's not overfilled.I was thinking that the evaporator is freezing up. I there some sensor that could possibly be bad.
Very Hot In Ohio

 

Check to see if the compressor shuts off when it starts to blow warm. I believe the 2nd gens have an overload sensor, maybe thats kicking it out.