Branson3520R, sounds like it needs what my manual refers to as "sensibility adjustment" (I think that is a typo as they call it sensitivity adjustment in the instructions below). There is an accompanying drawing I can scan if you want me to. Basically on the forward side of the 3pt (under the seat for me) there is a screw with a locknut.
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From the manual:
1. Position the rockshaft lifting arms, with a weight applied, so that they reach about half their angular movement range.
In this way the Control valve is positioned in its neutral phase.
2. Loosen nut "A" and turn the embedded hexagonal screw in a counterclockwise direction until the lifting arms start swaying continuously.
3. With a slow movement stop the movement of the arms by rotating screw "B" in a clockwise direction.
4. When the arms remain blocked in their position, which can be controlled by the relevant instrument positioned on the end of the lifting arms, rotate (clockwise) screw "B" for 1/2 - 3/4 of a turn max. Lock the position with the self locking nut "A".
5. In this way the Control Valve is sensitivity adjusted.
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Does this sound like it might match what is happening to you?