Haven't had the jerk-up problem. I'm assuming you checked your hydro fluid level? Sometimes the jerking is because of low fluid. My 2538 doesn't have a dip stick so I made one for it. I doubt if your 2638 has one either.
I made mine out of a long zip tie (because it's flexible), and measured down from the filler port on top of the tranny down to the site glass. Then transferred the measurement to the zip tie. Then checked it that way. I can't tell from the site glass alone if it's too high or too low. (I tend to error a little on too high)