I'm guessing this will fix your problem...
I had the same problem, cold air until the car got hot and then no cold air again until some time had passed. I fixed this problem two days ago. First I had the the system evacuated and recharged which did not fix the problem but was probably money well spent considering my A/C had never been serviced at all - 1998 Contour SVT with 108k miles.
To fix the problem I simply had to adjust the air gap between the clutch and the pulley. As the clutch wears the gap gets larger than spec and when hot the coil cannot pull the clutch in to engage the compressor. A simple test is to bang on the clutch with something when the A/C switch is on and the revs are elevated because the ECU thinks the compressor should be running. When I did this my clutch immediately sprang into action and the compressor ran.
This guy put up a nice how-to (for a Saab but the Contour A/C clutch looks exactly the same but with smaller shims) with good pictures and an explanation.
http://www.saab9000.com/procedures/ventilation/airconclutch.html On my car there was only one shim on the compressor shaft so I filed it until it was about half its orginal thickness and the gap was in spec. Spec is .014 - .033 I have run my car for hours with the A/C on in the last couple of days and it works perfectly.
I searched the forums for a solution to this problem and the closest I found was someone who replaced the clutch (in doing so he corrected the air gap which is probably all he needed to do in the first place, my clutch has plenty of meat left on it after 108k miles), I really think this should be a sticky because a lot of users seem to have this same issue. It can be fixed easily for free and about 45 minutes of your time.
