That is what they call a "hunting idle" in the Mustang community. Usually a bad or dirty sensor somewhere in the fuel injection. Most often a dirty MAF sensor, but we use a Speed Density FI system not a Mass Air system at least on the GT). I would see if you have any codes indicating problems. Could be an O2 sensor.
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 '03 Inferno Red GT, 5 speed, STAGE 1, Mopar BOV, Plastic Intake, Modified Upper and Lower Airbox, Custom Airbox-to-turbo pipe, Borla dual exhaust, Maddog Short Shifter, Drilled/Slotter Rotors, Ceramic Pads, chrome wheels, chrome door spears, AMP chrome fuel door, PT Cruiser 3rd light diffuser, H&R Springs, Rear Sway Bar added, MGW chrome door lock pulls, Blane's hood struts, Custom license plate mount, some interior dress up stuff...more to come
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