How could i stop the shifter on my 1994 firebird 3.4 from jumping up and down when i hit a pothole?

I changed the motor mount, mount as I was advised but it didnt work. I heard it was the thrust bearing (whatever that is) but the popular parts dealers have no idea what im talking about. The car drives great till i hit a pothole, then the shifter bobs up and down with a knock noise so then I have to accelarate to make the bobbing and the noise stop. please give me some advise