Sounds like a control pressure issue.

The transmission is mechanical/hydraulic. Hydraulic pressure is used for shifting and for compressing the clutches to maintain friction, if the hydraulic pressure is inadequate the clutches and bands will slip.

A competent shop can test the working pressure of the transmission, which IMO is the next step. Since the problem seems to be affecting all directions and gears, it is not a single clutch-pack, piston, or brake band (the pistons operate the brake bands and have nothing to do with reverse).

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