This could be a couple of things. First and foremost it sounds like a fluid issue. If you have not changed the fluid then let's start with this.
If that is not the issue then we most likely have an internal issue either with the control side or the clutch material in the unit is worn and is slipping when the fluid gets hot.
I am attaching the process from the manual that will help in replacing the fluid. Let's start with this and go from there.
Images (Click to enlarge)
Monday, February 22nd, 2021 AT 4:02 PM