My money is always on the solder joints in the main ECU/Fuel pump relay. Have you ever pulled it out and inspected or reflowed the solder joints?

The solder and steel(?) pins and rivets that goes through it expand at different rates. Eventually, the connection becomes flakey on rare occasions but good enough most times. It's a recipe for ghosts in the machine. The transition from good connection to no connection takes longer than anyone could keep the car running, so it will never completely fail. To bypass you can stick a pair of 14ga wires with spade terminals into the appropriate terminals in the relay panel behind the ashtray, or you can simply reflow and maybe add a little new solder. Reflowing is easier. I use a jewelers loupe to check connections for fun, but I go around and reflow *all* of them, even if I don't see obvious issues.

Good luck!