01 lesabre custom pauses then jerking into gear and jerking when stop

Hello, There are several causes that can cause this, and they surround the automatic transmission (not to state the obvious). There is a chance the transmission control module is faulty, but the transmission should be inspected and the vehicle test driven before anything else is done. I recommend a transmission specialist as they will likely have a diagnosis very quickly, and the issue is nearly certainly internal to the transmission.