sounds like the shut off valve is shot probably leaking inside the Injection pump its really hard to tell with the valve still attached
I would recommend changing the valve its a very easy job and if you get the part from www.performancemotorcars.com

it will only cost $33.09 cheapest your going to find I would think next runner up is this site for $10 more I would do a search on shut off valves here and get instructions on replacing. even if its not the cause that valve will go one of these days might as well change it while the parts are cheap
but that is my theory


Jeff