Why would a 2000 Pontiac Firebird transmission shift hard when driven for a while even after the filter and oil was changed?

1999 firebird doing the same thing... the trans has a support mount that has a piece of rubber that touches the trans to keep the trans stable. after time the rubber wears out and you will replace the trans mount. dont take it to the dealer it is costly and the part is around $40. you will have to raise the car up to exchange..Look under car to find the part its right between the bucket seats

good luck