I have a working theory.
I am recalling back to all the issues I've had in the past 3 years, and I think I should share these to aid in troubleshooting. #7 and #2 have been my problem children ever since I started analyzing the truck. I've had to swap #7 out for out-of-control cackle countless times, and #2 gets funky once in a while, then I've swapped it out maybe three times (total). #2 is building cackle right now, and it's been in there for about 6 months and 10,000 miles. I just swapped #7 (again) about 500 miles ago, it would miss badly when hot, and the #9 injector seems to have taken care of that. One time I was climbing a very steep grade at 45 MPH with 1100-degree EGTs, and the #7 cackle grew insanely loud in a matter of seconds.
We all know aerated fuel causes cackle, but my fuel system has been modified to avoid air in fuel. So... what if I'm still getting aerated fuel, but not from the usual suspects? What if the fuel is boiling in that long metal tube that runs down the valley and right next to...
wait for it...
the hot side of the turbo and the 4" downpipe?
I'm going to install some insulation on the fuel lines, but the tricky part will be to find a material that can take the temperatures near the downpipe. Anybody who remembers the old days of "Vapor Lock" will know exactly what I'm talking about.