Your symptoms are such that I have not encountered before. From what you have posted it appears that the problem only affects the hydro motor, not the pump. The hydro pump swash plate changes from vertical in neutral for full angel either way to give reverse and forward in the lower half of the speed range. That parts is operating correctly. The upper half of the forward speed range range is determined by the angle of the motor swash plate. The problem occurs when you start to control the motor servo. One cause I could come up with is that the motor servo is leaking all the oil out of it when it is not being used and then when you start to engage it the pump servo pressure drops off as all the oil flow goes to refill the motor servo. That would allow the pump servo to be forced back to a vertical position until the motor servo fills at which point the pump servo would be forced back to the maximum angle resulting in a jerk. from then on it would work OK until the motor servo was no longer in use and the oil could again drain out. Since the oil level in the hydro section is fill to the top when the tractor is operating the only time the air could get into the motor servo is with the engine off. That would explain why it only happens once each time the tractor is used.
When you start the tractor try putting the range selector lever in neutral and then pushing the speed selector all the way forward for a little bit before trying to move the tractor and see if that eliminates the pause and jerk.
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