On rare occasions, it is the fuel pressure regulator. Some of these have a spring that isn't centered. This causes the valve seat to wear only on one side. High vacuum conditions (foot off the gas)cause the valve to open more and it sticks open on the side of the bevel, dropping fuel pressure. When pressure tests are done, everything usually seems normal. If you don't have a pressure gauge to monitor the fuel rail (34# idle, up to 39# open throttle, engine dies at about 20#) the fuel pump fuse can be pulled (1/4" spade terminals fit in the fuse socket) and wires run into the cab to a 10A meter. Normal is about 4-5A and current will vary about 1/4A in driving. Engine will die when it drops below 2A. This will at least identify if it is a fuel problem.