Check Engine Light - throttle problems

What is your question?

Purchased an '04 in mid 2005 with 30,000 mi on it. Just barely touch the accel pedal and the car would rocket from a dead stop. Horrible gas mileage. After 6 mos. the check engine light came on and the car became significantly slower and stopped downshifting when attempting to pass or speed up quickly. I know the two are connected somehow.. whats happening?