ORIGINAL: RCPAUL
I'd suggest that the problem is fuel foaming in the tank. The tank may no have enough padding around. The symptoms are classic: when on the ground or held there is enough damping to prevent the fuel from foaming. In the air, the vibration increases foaming the fuel.
I'll look at that tonight. There is not much room in the tank compartment and I have foam below, beside, above and behind the tank but the way they make these stupid arfs where the tank neck sticks through a hole in the fuse, that puts all that vibration into the tank. There is not enought room to place the tank farther back and so I might have to bore out the hole in the front of the fuse to issolate it.