A/C Not Always Cooling

Hello every1 , I got Isuzu Trooper 1999 . its problem is : when i switch on the engine , the A/C works , but hot air comes out , Unless when i press the gas on , after some time it works , or when i am on a high way, but as soon as i stop for a trafic light , it become humid , and hot... So i changed the Compreser . but still the same probelm... So...Any idea , and 1 more thing , does the ECT Switch or A/C thermo switch got anything to do with that???? My regrads