Many years ago I had a split joint cup,the one at the motor end that connects the motor to the gear tower via the drive shaft,as the motor rotates bogie one is driving as it should correctly but because bogie two joint cup is slipping it causes the jerky running.for me the only cure was to replace the motor complete with tight joint cups,the joint cups look like they are pressed on and split sometimes.
Hope this helps with your diagnosis,of course it could be a completely different fault on your loco,that was the problem was with mine.
Cheers
Steve