My AC starts off blowing nice and cold, but after 5-20 minutes it it loses its cool (blows warm air) it is a 2001 ford escape.
Answer:
Look under the hood when it quits blowing cold and see if the accumulator looks frosted. This is common to a Sticking A/C Relay. To locate the relay, look in your owners manual under fuses/relays. Just replacing the relay should fix this common issue.
What is happening is the compressor is not able to shut off and causes the system to freeze and does not work again until is has time to thaw out.
Online Air Conditioning Information Resource