My 2005 Altima is having some idle patterns. 2days ago the car was in drive and I was waiting for the light to turn green and it was idling around 300-200 rpm and the car turned off. This was never a problem, but about 1 week I did install an aftermarket exhaust header and a cat back exhaust. I notice this idlying problem when the car is cold but if I put the car in neutral I'm good. Could it be that the oxygen sensor spacer I put on the headers be to far? Keep in mind its an auto tranny, I appreciate any help or tips.