Could be part of the solenoid pack is playing up, there are filters in the labyrinth which could be blocked. If the gearbox works for a short while, the symptoms point to fluid starvation. If the fluid that's in there is dark and stinky, it indicates a slipping clutch burning, again could be due to a fluid pressure drop, a cracked reverse piston will cause a pressure drop also, and will affect forward gears. This is under the first cover on the box and is very easy to change, with the gearbox in place, careful reassembly is necessary though.
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