A/C problems

My brother in law has a F-250, in which his a/c works fine when running down the highway. When we stop at a redlight it just feels like it quits working, any suggestions on what it could be. Other than just cleaning the dirt and dust. Any suggestions would greatly be appreciated.

Odds are it is just low on refrigerant, or the vaccuum servo is leaking/weak. my money is on needing a recharge.... at low speeds you aren't getting enough "head pressure" in the compressor.