The only thing not counted was that we think the mysterious reason was actually a vacuum leak in the secondary diaphragm.

I know it sounds like a fuel pressure issue, but how could a mechanical fuel pump possibly be getting that much pressure built up? Similarly, I shouldn't say the needle sweeps up and down the gauge. It looks more like a severe rattle in the range of 6psi to 15psi. It doesn't do a smooth sweep like a speedometer or a tach or anything.