2002 Mercury Villager Air conditioner

The belt wines real loud when I am driving in slow traffic but stops when go going at higher than 40 mph and today the a/c stopped blowing cold air and started blowing super hot air! I tried the knobs and everything I could think of but it still blew hit air. The back was also blowing hot air. I turned it off and rolled down the windows and had to get on the hwy. After I reached 60 mph I tried it again and it blew cold air again until I had to slow down to 50 and them it went back to hit air. When I git home my husband looked at it and he don't know much about cars that were built after the 70s. So he was no help. He put a little water in the radiator which was full. Anyway I needed to go to the store and tried the a/c again but still hot air so I turned it off rolled down the windows and headed down the road. Right before I got to the store I tried it and it was blowing cooler air. This really has me baffled! It blew cooler on the way home as well. But when I parked it went back to hot air again. Please help!