I have an 89 GTA which is a 383 with some upgrades. The car has 2500 miles on it and has been running fine. I was driving along the other day just enjoying the nice weather and right out the blue the car starting backfiring and really running bad. I drove it home about 4 miles and parked it to cool off till I could look it over. I pulled the plugs and decided to change them being I was taking them out. All plugs liked fine however the back plug on the drivers side was a bit darker than the rest. I decided to change the cap ,rotor and control mod. This bugs me because everything on the engine is new. There really is not much difference at this point. You can rev it up to about 2500 and it runs fairly smooth but you can hear a mis fire is there. When the car had about 500 mile on it the distributor went bad and it seemed about the same. Can this distributor be bad too? If so is something causing this to happen? Normally this thing has alot of pull and runs fine. What can I check? I did early that day add a fuel booster to the gas because of the crap fuel you get today.I can not believe this did anything. I drove about 50 miles with the additive and the car seemed fine. It was an STP octane booster. Please help.