My new to me 348 Challenge is cutting back, loses power and back fires, stumbles, once I hit 4-5Krpms. I noticed this earlier at much higher rpms, and was told that it was the upgraded chips that act as a rev limiter at 8600 rpms. The problem really seemed to begin in earnests once I filled the tank. I also noticed that air presure had built up in the tank when I unscrewed the fuel cap. What could it be? Once I lift off the loud pedle, it goes back to normal. Air flow meter? Fuel pump? filter? bad fuel? spark plugs? As you can see I need some experienced help. I assure you it will be appreciated.