From what I understand of the system you have.
the tank is of the piston type, it sounds to me that it could be an
electrical fault, either a loose wire, connecting now and then,
I have the same fault with a new switch, on my build, and it is now glued in so as to be water proof, but it is only for the nav lights and will do.
The other thing it may be, is what is known as a dry joint , caused by poor soldering, televisions also suffer from this problem, and happens in the factory.
get someone good at electrics to check it over.
I am no wizard at electrics, but this advice is based on, my own experience.
Hope it helps.