A/C problem
 

2006 F350 passenger van the when accerating up a hill or give it gas while on flat ground cruising along the a/c will stop blowing cold and just blow warm air you dont have to smash on it for it to happen but once you over the hill or back to cruising the cold air will return.Basicly when ever the engine is under a load. it just started doing this a few weeks ago i suspect a vacuum problem but dont know where to start searching for it.Where is the vacuum canister on this van?