It could be fuel starvation if the electric pump is not pushing fuel toward the mechanical pump.
OP: you could check the fuel pump relay with a good ear. Turn on the igntion to on, listen for fuel pump relay to drop out in 8-10 seconds. That would indicate that the fuel system should be pressurized.
Inspect the oil pressure sending unit. Look at the connector for any signs of melted areas or oil leak. Located on front right side of engine. My book does not go back far enough to verify wire colors
Oldest model shows orange wire should have 12 volts DC with ignition in on position. The tan/white wire should come out of oil pressure sending unit and output from the relay to power fuel pump. If you have a elper you cn disconnect at oil pressure sending unit. connect volt meter to tan/white wire at connector. You should get 12 volts while relay is energized.
You can jumper between orange and tan?white wire to see if you can hear pump run.