Sounds like the infamous case of throttle hang. My 05 does the exact same thing. Every time i get on the car real hard then imediately let off the gas......like if you take off hard in first or second hit 55mph and granny shift into 5th gear, you will have the rpm hang around 4-5k rpm. Its typical of the cars. Just shift quicker

On another note, i have heard this can be tuned out if you have a good tuner. It was for emission purposes