HAhaaa. been two years since I have deal with a fuel issue on a 206. I know some pilots just leave the fuel valve open all the time. Shouldn't matter.
But, it is a good bit of troubleshooting because assuming the electronic valve is solid, and you have it off right after shutdown, the problem must be below it.
I would focus in on the lines themselves that are submerged in the fuel. Could be a small leak in one of them, or it could be rotten. You would not notice because the pump would pressurize it and just piss the fuel back into the tank.
One way to test would be to drain the fuel until just at the top of the pumps, close the electronic fuel valve and turn on the boost pumps. Peek into the tank with a flashlight or just listen for spray or dripping.
This would explain the intermittent nature of the problem too. The leak would only matter if it was below the fuel level. So, you could even experiment in the mean time and see if it happens on a full tank vs minimum fuel.