Here's my problem:
In 4th gear, pushing 3000 rpm, I start to pick up a misfire, and in 5th gear I will also get it under heavy acceleration (passing or going uphill). A friend said he heard it misfire when downshifting, but I cannot confirm that due to my exhaust tone/rumble. It is not throwing any codes.
I have a 2006 Mustang GT, and the only mods are a CDC Shaker and K&N air filter.
I have tried to fix this by replacing the fuel filter (turned out to be the original from 54,000 miles...

2006 Ford Mustang GT Misfire at 3000 rpm