Make sure the tail blades are 90 degrees to the main blades, if they are off this can cause bobbing.
Also check the tail gear box to see if the alignment pin has sheared off.
If so drill out the point where the pin was and put in a short FAT screw
You only want 1/8 to 3/16 inch showing inside the gear box.
This will go into the tail boom alignment hole andkeep the tail gear case properly aligned.
If you need a new case here is a link for you.
https://www.ushobbysupply.com/produc...roducts_id=129