ive had problems from the pressure being to high. one of our "new guys" at work opened the bottle with the flow adjustment on the regulator. that lil ball in the glass site gauge bounced off the top with a solid thud ...
i didn't realize this till i had started to lay a bead in one of those "impossible to get to with a grinder" type spots
IIRC to much flow will suck air into the shielding glass from around the edge of the nozzle, diluting the shielding gas and causing porosity, just like the venturi in a carb sucks fuel into the air stream
a properly setup mig welder should be able to lay a porosity free bead even with a light breeze blowing across the work piece. if the smallest movement of air causes porosity examine the entire sheilding gas system for problems, THEN play with the pressure / flow settings