Nice work Mike. In doing it, you've probably jiggled the loose or corroded (heat producing) junction enough to keep it from failing for a while. That you seemed to have changed things, makes it more likely you're in the right area.

Based on the memory of those charred 740 fuse blocks I've seen in the yards, I'd guess the problem is in the female connectors of the relay's socket. I'd be out on a limb here, but I would also guess a pro's solution, to avoid the bringbacks, would involve replacing the fuse/relay block, relay, and harness ends.

That would be kinda cool, though, if you saw the meter take a dive, then saw a corresponding dip in fuel pressure, all going back to normal when you wiggled the plug end of your extension cable.
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Art Benstein near Baltimore

A closed mouth gathers no foot.