Air Conditioner does not cool when engine is hot

[dsira]

1993 Continental with original air. Just last weekend the air from the vents was noticiably warmer than it normally is. Early in morning when car is cool the air conditioner will put out some cool air. Once everything warms up and the car has run awhile it is barely cool. The shop said there is freon is ok. The compressor is kicking in and out like it should, although it seems to kick on and off every 20 seconds when things are hot. The vent doors seem to work when I select vent or defrost, moving the air wher they should. The controller seems to be working as far as I can tell. Any ideas?