Still have no idea what the problem is. It's rarely risen above 80 degrees outside since I returned home, so I haven't been able to cause the issue to come up again.

But, I did finally get that fuel pressure gauge on! Wish I could change the orientation a bit, but I already had to tighten it way more than I was comfortable just to get it to stop leaking.

 

That's because it's a fitting for a hose nut, not a pipe thread! The hose nut has a pin in it that holds the schrader valve open too.

 

The rail-end of the adapter is a schrader fitting, and I took the valve core out. It was actually leaking between the adapter and the gauge. I added a small amount of pipe joint compound on that joint (don't think it would hurt the injectors, but also don't want to test it on mine), and that helped some. Still had to torque it down quite a bit though.

 

I think you're looking for a pressure around 48 when running.. The needle will vibrate some.
The important test though, is the bleed down test. It should hold pressure no less than 25 for a certain number of minutes after you shut it off..5 I think? Maybe someone else knows.

 

Ya, pressure is good while the issue isn't present.

48 PSI while running
44 PSI immediately after shutting off
46 PSI after 5 seconds
37 after 5 minutes
32 after 10 minutes
20 after 15 minutes.

The little rise a few seconds after shutting the engine off is interesting.
The big drop between 10 and 15 minutes is probably due to whatever kind of check valve it has being less effective below about 30 PSi downstream pressure

Will probably have to wait for summer and another long trip to be able to get a reading while the issue is ongoing. Haven't found any other way to replicate it.