2003 Mustang GT bogging down

When was the last time the plugs were replaced? I'd consider the PCV valve as well or possibly a vacuum leak that shows up at higher speeds.

How does it run when under high load? Burning tires, dropping the clutch ect. A failed oxygen sensor would cause several issues. Your mpg would suffer dramatically since the pcm would have no idea as to what your airflow ratio is. Signs would be a strong odor of fuel. The pcm makes corrections based on your oxygen sensor. When in 5th gear hitting the gas and it doesn't go would lead me to believe its getting too much fuel or not enough causing the issues. Have you check your codes? How many miles are no the car? Has the oxygen sensors been replaced before?