poor performance; lacks power and acceleration

hi, i was wondering if someone could help me out with a 98 3.8 v6 mustang with 124,000 miles on it. It has poor acceleration and lacks power to the point where its speed drops no matter how hard i accelerate when going on a hill. ive recently changed the oil, fuel filter, and installed a CAI on it in hopes of gaining some power but it hasn't fixed the problem. :disappoin

Spark plug and wires been changed out yet?

plugged converter is my guess