99+ GT's have a little bit of a rough idle. A aftermarket chip should smooth out your idle. Stock there is no "chip". There is a computer that is programmed. You can either flash the computer or add a chip. I believe on the 99+ the computer is on the passenger side behind the kick panel. I could be wrong though. I have had a 2001 and now a 2003 GT and the both idled slightly rough, not bad though. I asked the same question and was told that in 99 Ford started using an agresive cam lift( Camshaft lift: .505/.534-inch lift Duration (I/E) @ .050" 186/194 ) which contributes to the slightly rough idle.