you are probably getting low on freon causing your compresor to cycle on and off quickly. when the compresor is on the pcm raises idle to compansate for the load and drops the idle with the compresor off. the most common ac leak is the accumulator that is located on the sub frame under the battery tray. i just replaced the accumulator on my brothers 96 'tour. most r134 hasa dye in it and should be visible with a black light. a GOOD repair shop will have one.
