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.

When you were in the air box in what position was the flap? Several months ago I replaced the air box thermostat, cleaned the box, and lubricated the door mechanism... to this day I still feel the flapper door is a major weak link in my air induction system... and what I finally believe to be the result of this annoying mid-range ping.

Personally, I plan to gut the air box and modify the heat exchange inlet (from exhaust manifold) with some crafty fiberglass/paint work.

I am not saying this is your solution, but check the air box door to see if she is opening and closing fully.