loud bang while shifting and shake and check engine light kicks on
 

i have a 00 lumina. started to shift hard while warmed up and has been back to the tranny guy 2 times now and once to replace the whole valve body bc i knew about the pcs being bad and the whole valve body was replaced. 2 weeks later started shifting hard again this time thought it started to shift hard then it got considerably worse when it made a loud bang into gear and shook a little afterwords. It also threw the check engine light on. Not sure what the code was but took it back to the tranny guy and he replaced the torque converter this time. well not 4 hrs after having it back its now shifting hard, then once its really warmed up it shifts with a loud bang and gives you a bit of a shake then speeds up to the next shifting time and does the same thing again. Ive noticed too that the rpms never seem right when it starts to shift with the bang. I could put my foot through the floor boards and still get 15 to 20 rpms which i know is not right. and it wouldnt want to pick up speed much really either once its banging. its thrown the pcs code every time it comes back from the tranny guy and im getting fed up with this dime a dozen car but its all i have for now, its nickel and diming me lol . but can anyone help me and tell me why it shifts hard when its warmed up and the longer i drive it is when it starts to shift bang shake...as well as kick the check engine light on? any help is appreciated this has been a 6 month on going process with this car ........