Will not down shift

The problem I am having is the car will not downshift from overdrive to third gear when you try to accelerate, when it finally dose down shift the RPM's go up and it feels like the transmission shifts to second and starts jerking like its stuck between second and third. Pull over and stop then take off the car will shift normal. I took the car to a transmission shop and had it rebuilt. I have taken it back to them twice now for same problem. They are telling me that it is a motor problem. I have jumpered out the two pins on the OBD1 connector, I get code 12 no other codes or check engine lights. I check all the fuses no blown fuses. Checked the connector on top the transmission no burnt terminals. What is wrong with this car?