As said I would suggest changeing the filter first. EDIT then I would take off/check/clean the 90 deg filter adapter if it has one on your app. then I would preoil with it still off & see if it gushed out the hole especially if you can preoil it at a time when it has zero psi & all that is left is the pickup plugged (likely at that point) or pump bypass. I'm thinking clogged pickup but holler what it ends up being. With a flat circular plate you might also preoil it & see if it flows out the small holes. Actually I would grab a helper & have him undo the filter/plate/bypass while you are preoiling & do this at a time when it is showing zero psi, anything to avoid pulling the pan if you dont have to tho I ain't thinking of anything up top that would make this intermittent once it makes the horizontle turn back to the vertical feed.