I have had a problem where before the car gets warmed up it will hesitate and backfire a little, then it will run fine. I have gone through the normal checklist of things, it has new AMM(I think backfiring killed the old one), new FPR, cleaned IAC and throttle body, etc. Then when I thought back to when this first started, I had just replaced an VERY dirty air filter with a new one. SO there must be a difference in air flow, does the ECU instantly compensate? Could changing the filter cause this type of problem? I am going to replace all the vacuum lines and perhaps the coolant temp sensor next.