96Deville idle climate control problem

I recently purchased a 1996 Caddy Deville with 10,000 original miles from my wifes 95 year old grandmother who bought the car brand new in 1996. The car is great...except... after driving on the highway at 65-70 mph for 20 min. or so, several things get crazy. The idle speed will stay high at 900rpm in gear, and the AC compressor will disengage and the auto climate control will blow warm air even though it's set to 72 and it's warmer then that in the car. At this point you can turn the climate control all the way down to 60 and it still blows warm air and idle remains high, and windows inside start fogging up, even though the settings are not on defrost! If I stop and run into the store for a minute, when I come back out and restart the car it seems to reset and all works fine again untill my next trip at highway speeds! The problem does not occur during slow around town short trips, but does happen everyday on my highway commute to and from work! Any ideas for what the problem is? I assume the idle staying high and the ac compressor being disengaged are related to the same problem because they always happen together. Thanks in advance for any help.