one of three reasons; either your float height is wrong, or you have a crack in the overflow tube. But, if its only intermittent, the most likely culprit is debris in the float valve.
Is you fuel tank clean and de-rusted? Small particles of rust leaving the tank will prevent the float from properly seating and cause this issue. Do you have the in tank filter installed? Are you running an inline filter instead?
Before you set fire, call me, I'll come pick it up, fix it, and sell it back to you