A/C Gets Hot When im moving, Cool While Idiling???

My truck has been doing this for a while...and its starting to get realy hot so i need to gget it fixed! if im idiling it blows out cool...not "cold" but good enough. As soon as i start going down the road it blows out hot air... I bought a freon refil bottle today with a pressure guage on it and it says start the enigne and run the a/c on max wide open. WHen i hooked up the bottle the pressure went to the red zone.. the bottle says that if it IS in the red zone dont .fill it up and somthing else is wrong with the system. So I figured i would let some pressure out and refil it with some new stuff. Got the pressure at the right PSI and went for a drive...got cold for about 2 minutes then back to hot. Checked the psi when i got back and it was wayy in the red zone...any ideas on what the hek is going on here??