2004 Chevy Impala LS- Runs rough, added gumout to gas and Even worse now.?

I had to replace my injectors when my car had 36,800 miles. My car now has 66,000 miles. My car started to run a little rough. And had almost the same symptoms as original. I decided to put a can of Gumout fuel additive. I was on the highway when I noticed my car gettin feeling gutless. I decided to accelerate a little more, in hopes of pushing the additive through the injectors to clean them out. The car started sputter even more and I was unable to accelerate. I took my foot off the gas and slowly began to apply pressure to the gas pedal. It would accelerate and pick up speed. When I came to a stop the vehicle was running very rough. My RPMs were going from 1000 to 500. I ran the code finder and discovered a P0300- a random misfiring. We just gave this car a whole tuneup. I decided to add a whole complete set of new fuel injectors, and a fuel pump. Now the exhaust does have a funny smell, but I'm having a hard time believing it would be this. I am a nurse not a mechanic. We ran the vehicle and it's still rough. When you begin to accelerate its as if it's hesitated. But eventually it will go. Any ideas or suggestions. The car itself shifts great, oils been changed routinely. I've cleared the codes, but they return. I've checked the spark plugs, and they look good. Please help!