slow stang!

it runs smooth until about 4 grand on the tach then it drops off. throttle response would be normal to a little hesitant. I have no sensors or codes goin off...im waiting on a new intake to come in cause my air filter is for crap. But it feels like a really heavy car.
 
ohh and the latest is when i get on it and kick the rpms up high and then let off the gas to an idle it stops at 1000 briefly then drops down to about 500-600 shakes for a few seconds then calms down back to 700-800 rpm