My guess is a blocked vent line...either with an insect next (wasps seem to love them) or with fuel collecting in a low point in the vent line and blocking the air. Disconnect the vent line at the tank and try to blow it out...If that is it...trace the line to eliminate low loops and future problems. If the tank air can't escape as you are filling it creates surges backup the fill line and gives the impression that the tank is overflowing.
No longer posting. Reach me by PM!