Briggs have two in-line fuel filter types: a type for gravity feed, and a type for fuel pump feed. If you fit a pump-type filter in a gravity feed system, fuel starvation does occur at full load.
I think you need to remove the filter temporarily (substitute a short piece of tubing of the right diameter) and see if the symptoms go away. If they do, either fit a new fuel line or see if you can get hold of a lower-resistance filter. If this does not solve the problem, please tell us, and we can dig deeper.
