1989 Chevy Camaro Tranny problums

I have a 1989 Camaro RS with a 305 TBI with auto transmission, the car has sat for a few years untill I got it this last weekend, and when I goto move the car It will take a few second to actually kick into gear when I put it in, also when im driving I give it quick burst of gas I hear and feel a loud thud, why does the tranny do that, and how do I fix the problum. Does the tranny need to be fushed out?