Acceleration issues.

tryingtofix

I have a 2004 Pontiac Grand Am GT with the 3.4ltr engine. About a month ago I accelerated quickly and the "Service Engine Soon" light started flashing. After a few seconds and getting back to normal RPMs the light quit flashing. Since then it has got progressively worse. Now, under the same circumstances the "SES" light would flash and stay on. The light usually went out after a day or two, as long as I didn't accelerate hard. I ran codes and found "P0300", Multiple Cylinder Misfire and "P0306", Cylinder 6 Misfire Detected. While driving, once it shifts into the next gear it hesitates. It does this through all upward gear changes. I can press the accelerator more and it will rev-up and eventually get up to speed, but it wasn't like that before. The gas mileage has suffered greatly too. I've replace the spark plugs and wires and about the only real difference is that the "SES" light no longer comes on when I accelerate hard, but the hesitation is still a problem. I'm not sure what to check or have checked next, any suggestions? Thank You!