We've got one parking spot on our ramp that kind of slopes to the left a little, and if you leave the fuel selector set to both, it will crossfeed due to gravity. Leave it overnight and in the morning, the right wing will be half empty while the left wing will be full because the fuel is leaking from the fuel vent(due to gravity) and will be continually replenished from the other wing. I don't know if this is the problem you are having but you might try turning the fuel selector to either left or right, or even off, when parking for extended periods and see if it does the trick.