2000 Dodge Caravan Will not blow cold air sometimes other t

For the last 3 years or so the air cond has not been working properly. It will blow warm air every time you cut it on and does not cool. Then all of a sudden, you may try it and it works great. So cold you can't stand it. But most of the time lately, it just blows hot air. 2 years ago, I had a new radiator installed. Lately have been having problems with car running hot. Strange. When the car starts to run hot, the air cond. Gets cold. I have had the AC serviced and told them about situation but know one knows what to do. Help needed just to know what I should do. Thanks