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This could be caused by internal seal leakage in any of the various hydraulic circuits, incorrect clearance in a clutch pack, low line pressure, or a faulty (sticking) solenoid.
In my experience, Subaru transmissions are very sensitive to dirty fluid. I would suggest doing a transmission maintenace service: change the filter and flush the transmission to exchange ALL of the fluid, including what is in the torque converter and the cooler. Often, this helps dramatically. Adding a can of seal conditioner (which softens rubber seals, making them more pliable) may be beneficial, but is optional.
If this does not help, pressure testing and transmission disassembly are probably going to be needed to locate the source of the problem.