Help diagnosing 2002 ford mustang v6 3.8l?

Check engine light comes on and off not simultaneously. One day it will be one, and next time i crank it the light is off for days. The battery, abs and trac off light stays on though. Even though the check engine light is off i can tell something is wrong because the car doesn t drive like when i first got it. When i take off for speed it feels heavy like it lags and feels like its pulling back. Sometimes with my foot off the pedal it ll pull back then push itself like shifting. Another thing i noticed is that after 80 miles it shakes a bit. That s all i can think off. If you need more info let me know.

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