Helped a friend with the same issue on an LMM just a few weeks ago. Once the fuel gets hot they seem to be bad about returning to much fuel.
I had an LBZ like this too. It would run great on any tune that was in it, pull a bass boat for hours on end just fine, but hook it to a camper and hour and half down the road she would be in limp cruising at 60mph. It would hold rail pressure just fine but if it needed to speed up, desired rail would go up and actual was very slow to follow, but would be climbing. Once the fuel got over 180* or so, it would get to slow on actual following desired, and throw the code.
I had input on a thread around here somewhere with part numbers.
I put a derale(sp) cooler out in front of the stack, inline before the fuel filter on the supple side and it has never happened again. I logged it some afterwards and the fuel never went over 160*.