Why is my a/c cooling intermittently? Cold upon first cranking only to get hot?

What is your question?

A/C is cold when first cranking..after 15-20 minutes driving it gets warm. May blow cold again and the get warm again at different times. Radiator flushed. Freon checked. Sometimes reads hi on freon. A/C also seems to come out defrost too? It started doing this about a year ago but not enough to really bother me and now can't go to far without it getting warm and it is driving me crazy!! I appreciate any help you can give me........