A to the K

My 97 Mustang, which just had a Hypertech tune, seems to backfire when power shifted. Any ideas of why this is happening, I am losing lots of power when this happens. List of already Mods below. I only found this out after the tune, it could of been from before but never noticed. When shifting normally the car does not backfire.

St4ng07

o/r pipe = no cats would cause it if im correct. Also wouldnt powershift at all..especially with these trans if you wanna buy a new one go right ahead.