Re-check the drive shaft roll pin at the pump input. Just because you can see the pin in the coupling doesn't mean it's not sheared where it goes through the input shaft. You can't tell just by looking at it.
Look at the rear of the hydro under the seat, and see if the input shaft, where it comes out of the backside of the pump, is turning consistently with the engine speed. If not, then the roll pin is the problem. If it is turning then could be trunnion or (much less likely) pump internal problem.
Second the recommendation to change the fluid and filter. Might be getting some air entrained in the fluid.
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