does the condenser fan come on when you turn the A/C on? take a look.
what happens is that the pressure goes high from lack of air across the condenser tripping the pressure switch and cutting the system off. then the pressure normalizes and the system cuts back on. off. on. off. on. when it is on it loads the engine more than usual, causing a low idle.
you're A/C should still work fine cruising down the interstate but lacks cooling below 45mph and idles funny sitting still, cutting the compressor on and off.
i've discovered the same issue with my '96.