i am soory to say but no body gave you the solution..
There are couple of thing sthat needs to be done.
You A/C Works dine when you car is in motion and poor when idle...If So..
Then tell me if for how long it trips, in idle....
LEt me give you a proper solution.
Try the following:
When the outside temperature is hot, and at idle your A/C is not working, turn on your A/c, and wash the radiator thoroughly and when you are sure that radiator fins are all wet, get inside the car and check for cooling ....check the difference...
If this works..tell me i"ll give the solution.