I have seen worn out keys do this. How about getting a new key made? Also, is your key bent in any way shape or form? This will also do it. A new key is $60+ at stealership or $30 at Ace Hardware, but at Ace, they duplicate you key, so if it is really worn, you will get a new worn key. Dealership "should" be able to cut one by your VIN.
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator