I'd like to chime in, if I may... and I KNOW I'll offend someone here..
It's your converter. I have the same gear/tire combo as you and run 122mph at 6400 rpm in the heat. Converter is nothing special.. just a $600 TurboAction S-800 med.
I have seen this more times than I'd like to count... unless it's broke, you've got a junk converter. converter is Sooooooo inefficient, you loose all your ET in the back half cause the car don't mile-an-hour good.
PLEASE, save this scenario for the phord/shivy guys.. I beg you.
Dynamic Converters is just down the road from you. They'll treat you right, have decades worth of experience, and you'll think you added 100 more hp.