Mechanical fuel pumps suck a lot harder than the electric ones do, it sounds like maybe you have a fuel problem with vapor locking I would check the fuel tank pickup filter and sock and all the rubber lines in the fuel system for possible leaking under suction I have seen the stock pickups(in older cars) be plug up inside them with solid guck

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)