Evil, I'm watching this thread with great interest. This sounds identical to what many of us with the Lund tune on the Sport are experiencing. I don't have any insight at all other than what I've experienced - pulls are generally clean if I go from a stop or a roll ABOVE 70 mph. Going WOT at a 65 mph cruise hits a big flat spot, sounds very similar to what you're experiencing. I have the Lund 93SS tune, 170 tstat, gapped plugs to 28.
It's more severe for some of the guys running the Lund tune in that the car has actually stalled or gone way down on power when going WOT from a 65 mph cruise, mine isn't that severe, and I didn't notice it when it was warmer, but now that it's getting cold up here in MN I notice it even more.
I'm traveling for work at the moment but when I get back I'm going to flash it to the 93 and then 91 DD tunes and see if it happens with either of those. Lund is aware of the issues and is supposedly working on something, but I'm not sure he has the data he needs being in a warm climate.