This is a long shot....take the gas cap off....see what happens....I had some crap like this happen to me back in the 70's and the lil "breather thingy" in the cap was plugged caused a world of **** until I found it....cleaned the lil hole out with gumout carb cleaner and WHOOP no probs.... not sure of your year you might have the return hose set up...anyway what do have to lose?
