I've seen my gauge mounted on my 2-port reg. go pretty low like that at idle as well with a mech. pump. How long is the line to your gauge? Maybe when the bowls are filling the fuel could back down the gauge line? Maybe try taking the line from the hood and watch a gauge sitting on the floor? If the car isn't running out of fuel I have to believe that the pump is fine. They are a pretty much work or don't work kind of thing. My .02...