Ford Repair: air conditioning, vacuum leak, compressor clutch

Question
I have a 1997 Ford Escort and the air conditioning is running
cold for 5-8 minutes and then it draws in the humid outside air.
I took it in to the shop and they couldn't find anything wrong so
they just filled it with freon (1 pound) and sent me home. He
said he tested it in the shop and it was cooling fine but it still
only cools for the first minutes I am driving. If I stop and park
the car say to run into the store and come back out again it runs
for the first little while then draws in hot air.

Answer
If I was diagnosing this problem, I would check the air gap on the AC compressor clutch. The gap may be too much, which when hot will not allow the clutch to engage. This is a magnetic device and heat has a tremendous effect on it especially when the magnet cannot overcome the air gap.
Are you sure it is the AC and not something like a vacuum leak?