1) Are you sure the leadouts are in the right position? If they're too far aft, the fuel runs to the front of the tank. The engine hickups, the nose comes in, the fuel has moved to the back of the tank, etc., etc. Fix the LO's!!!
2) If you can, shim the back of the tank outboard.
3) If it's a clunk tank, taper the clunk so it picks up all the fuel.
Some combination of these will most likely fix the problem.
Steve