If your jeep came with the tow pack then it will have the Engine driven fan and the electric fan. If you have both then Id say its not a fan issue. I would hope they checked the condensor for debris but check it just to be sure. Spray it out with a water hose really good, do not use high pressure. If its good and clean then i would guess its a bad compressor/accumulator. I do not know why they would say its a bad computer? Does the engine heat up any at all? Sometimes when the engine heats up the a/c will kick off to give it the extra cooling.
The reason I say it could be a bad compressor is, our 94 grand does the same thing and thats what we were told. You can hook a set of guages up to it and tell if the compressor is going out. The high pressure side will be lower than it should be b/c its not building up the pressure it should. The hotter the engine gets(Idling alot) the worse it will get.