1991 Pontiac Firebird engine runs rough

I have to press the gas halfway down for the rpm to go up and for the car to start moving. Sometimes it doesn't start moving until I hit the gas all the way down and then all of the sudden the rpm will shoot up to about 3000. Sometimes when I hit the gas it chugs really hard and sometimes it dies. When it is in nuetral I get a quick throttle response but when it is in gear it runs bad. Had it in 3 shops and they told me its running fine!