Before the tail jerking.

Did you have a crash ???
Did you have a 3 AMP ATO Fused installed between the Rx and tail motor input ???
Did you check the meshing of the tail gear to thetail rotor gear ???

The typical tail motor will last about 30 flights on average and maybe that is a sign the motor maybe failing.

Try this first, add more tail power by adjusting the tail gain. Additionally, if you are getting tail jerking, the gyro gain is too high and adjust to the "-" on the Rx with the power off.