Does it only do it with the defrost on or the heater, too? My car had the same problem - idle was erratic with the defroster on and would alternate between 500 and 1000 or so. The defroster uses the A/C to dehumidify the air and it turned out on mine that the A/C was low on refrigerant so the compressor kept kicking on and off and causing the idle to change. Mine was low enough that I could tell myself by turning the A/C on high and feeling that it wasn't blowing very cold.
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Strike down the unroadworthy!
2000 GT Atl. Blue: '03 Cobra motor - 465rwhp/473rwtq, T-56 6-speed, full MM TA/PH & tubular K-member suspension, Saleen body, 17" Torq-Thrust II's, M/T ET Street radials, and lots more
'94 Rio Red Cobra: All the bolt-ons, 3.73s
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