Sideline thought (don't think it's been mentioned above) - fuel icing (waxing) in cold soak conditions at cruise.
Recent instances which come to mind -
(a) airline B777 over the pole into the UK with a failure to spin up on short final.
(b) corporate turboprop Aust-NZ with a fuel tank transfer restriction in cruise - a jumpseating pilot with relevant prior experience diagnosed the symptoms correctly and saved the day on that occasion.
.. I'm sure that there have been a variety of others, of which I am not aware, in recent years.