at 4-5Krpms, my new to me car is begging to cut back and die...just loses power, sputters, backfires, then when I lower the rpms, it goes again. This car has race exhaust, and is a challege car with a fairly new engine. I noticed this at much higher rpms earlier, and was told it was the upgraded chip acting as a rev limiter once you got above 8600rpms. What gives? bad fuel? I noticed this happend after I fueled the tank. Also, I noticed that when I removed the tank cap, air rushed out. Air flow meter? Fouled plugs? Fuel pump? Dirtly filter? As you can see, I have no clue so your suggestions will be appreciated.