Mark,
I can't really say definitively, but in talking to my sources my gut is saying that you are seeing the injectors begin to fail in the ball seat area. This causes excess return rates and of course high fuel temps. I'm hearing more and more of this same complaint with the LBZ these days, but have yet to see one through to the end.
The big thing I am interested in is seeing a set of used injectors from a vehicle exhibiting these symptoms. Then we could test and look at whether it's an abrasive wear issue or a cavitation issue.
I have a notion it's time for another round of me harping: FILTRATION, LIFT PUMP, ADDITIVE...