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If you're confident that the selector linkage isn't worn and allowing miselection then I'd say its possible that the neutral to 1st solenoid is faulty or clogged or there's a lack of oil pressure to hold things in properly
So I'd begin by draining off the fluid and dropping the sump to clean out the filter screen and then try again with fresh fluid
Some manufacturers recommended a change of the type of fluid - Speak to the local dealer and parts dept - they will advise you on the updated fluid to use..
If that’s all OK then check all the electrical connections of the gearbox, remove each connector in turn and spray with contact cleaner. Check that the wiring to the gearbox is in good condition with no chafing or frayed wire
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