Stammering engine

Hi. 

I've had my 2012 sti. Rebuild with a forged engine and then remapped too 370.  This was done as I believed the stammer could have been ring land failure and after a diagnosis the car was down on one cylinder. The car however still is stammering when accelerating between 3 and 4 thousand rpm. The only other thing that used to happen was a air noise when changing gear that sounded like a Bov however I dont have a bov everything was as standard. The problem with stammering only happned after a service when the air filter was changed. I've changed back to a kn filter pannel. The problem is still there. Does anyone have this issue and know the answers. Spark plugs changed, and complete overhaul of engine leaves me wondering what else it could be.

Tha ks