Air conditioning problem on 1993 Ram Charger

I recently purchased a 1993 Dodge Ram Charger 4X4. The A/C blows cool air (not cold) for about 15 minutes, then it blows warm air. I have taken it to the shop and they put 1 can of refrigerant (freon substitute) in. It is still the same. I have noticed that the compressor stays on all the time and does not cycle. Could the A/C be freezing up? If I turn off the A/C for 10 minutes or so it will cool again for a while. It works OK in the mornings and at night, but in south MS during the day you might as well roll the windows down and go 70 everywhere!

Any suggestions?
Thanks