2008 6.7L 'fuel derate' and no codes

[Rogan]

sub 10k miles on new Bosch OE injectors
RaceMe Pro tuned (60hp setting, #3), deleted.
5" FloPro exhaust

For a long time, now, I've been getting a fuel derate under heavy throttle loads (like climbing a 4-5% grade in 5th or 6th) at like 40% or more throttle percentage.
As an example, logging with my XTOOL A30D scanner:

• 60% throttle
• 5th gear
• about 55mph
• loaded or empty, doesn't matter

Fuel pressure requested set point:  21000psi
Fuel pressure Sensor:  15000psi (more throttle decreases this pressure, I've seen it as low as like 8000psi)
HPCR Fuel Pressure:  follows fuel pressure sensor

After about 5-8 secs of this gap in requested pressure, the Derate Counter increments by 1, the engine gets flat (100% throttle makes no change) and the overhead console dings "See Dealer".
no codes are stored...

Things I've done: 

• I've updated the tuner's firmware
• re-flashed the truck
• replaced fuel filters
• replaced the Fuel Pressure Sensor
• Replaced Fuel Pressure Relief Valve with a Plug
  • Re-shimmed OE FPRV
  • Replaced with Bosch new FPRV
• Changed to a lesser tune, as well as a higher tune
• tried stepping up the rail fuel presser
  • no change, set back to stock

Last thing, I've ordered a FLEECE 170 lift pump, that will be here tomorrow.. 

The 'NO CODE' part is a MFer, since that gives me absolutely no direction.

 

Thoughts?  I'm at a loss, and I've been chasing this issue for a very long time.