Fuel.........supercharger will cause it to fall on its nose. But, I think you have a fuel problem. Supercharger pump failure will cause it to buck as well, kicking in and out. You stated it just falls off. If it is fine when cold, but does this when it gets to temp, then the pump may be the cause. If it runs strong up to 120 and falls on it's nose, then I would say a fuel pump, or fuel restriction problem.