for me the problem started exactly as you describe and then it would clunk back into gear which may sometimes cause the car to go into limp mode. You then turn off and turn on and problem would go away.
Crank sensor is sending engine speed signal to gearbox and if there is lets say an error in the info the gearbox receives then I am assuming a mismatch of speed and gear ratio is detected - hence the default to safety mode.
Changed crank sensor and cleared codes last Jan/Feb and clear since - but also changed ATF and filter prior to it happening the last time. Either way I have thus far avoided a far greater expense and time committment if had delved deeper.
Other causes can be a gearbox solenoid and bits of thrust washer in ATF which you need to look out for when draining gearbox.
Has yours been OK since we took a look earlier in the year?
I'm in Camberley on Saturday by the looks of it - be happy to check it again for codes if you like?
ATF fluid does apparently expand as it ages. That sounds like a lot, though. Perhaps it was overfilled, although I can't see that causing it to malfunction.
The other possibility is that (e.g.) the fluid pump in the gearbox is faulty meaning that it is not keeping the torque converter and other parts of the gearbox full of fluid therefore more fluid sits in the sump than would be normal with the engine running.
What condition was the fluid in? Nice clear red colour or burnt brown? Any black particles in the fluid?
I would try the cheap fixes first. ATF change and correct the level, looking for any bits of thrust washer in the sump, then change the crank sensor.
Kevin