2006 Ford Focus Intermittent Cooling

My A/C began to blow hot air while stopped at a traffic light and would get cool again when I accelerated. The Ford Dealership told me that my system was overcharged; however, I have never touched the air conditioning system. They did not offer a reason as to how it could have been overcharged, but they did adjust the pressure and sent me on my way.

The air conditioner worked great for about a week and the problem has slowly begun to come back, now the problem is reversed, when I stop at a light the A/C is really cold and when I accelerate it gets warm for a few minutes before cooling back down.

My engine hasn't been overheating.