'91 Econoline 150, Fuel Pressure goes low

Engine starts, runs good for about a minute and then suddenly stalls. Have to wait a few minutes to start again otherwise it cranks with no start. I connected fuel pressure tester. The fuel pressure builds up but takes longer than it should - have to switch ignition on and off a few times to get pressure up. When engine is running, it settles about 30PSI. After about a minute, the pressure begins dropping. It takes about 2 or 3 seconds to get down to about 20PSI where it stalls. I can hear the high pressure fuel pump (the one on the frame) humming for a second or two after engine completely stalled. I pinched off the return line and pressure build up time doesn't change and doesn't go any higher. After building up pressure and turning engine off, I pinched off feed line and the pressure leaks out too fast (about 10PSI in 15 minutes). This is too much, but it's not bad enough to stop the engine from running - probably some leaky fuel injectors. I had a helper switch the ignition on and off several times while I put my ear to the gas tank filler tubes. I could hear a single click each time time the ignition was turned on, but no buzzing sound as though the pumps are running. The fuel filter was already replaced by someone else. I believe that one of the pumps is faulty, but don't know if it would be an in-tank pump or the high pressure pump on the frame. I'm guessing that the in-tank pump is bad. I'm interested in hearing the opinion from those who have experience with this and any recommendations on what I should do.