Sounds like the tune is holding it in a lower gear when you accelerate hard....might try letting up just a bit at the 120 speed to see if it will shift up to overdrive after hard accel.
Panama City, Fl.
I wake up in the morning with nothing to do and by supper time, I have half of it done.
2000 Lightning, s/c, tuned, Basanni, built tranny
Sold them all.....
Previous Mustangs:4 - '66 Coupes(silver/blue/grey/red) '87 GT HO, '00/'02/'07 GT Vert(red)(grey)(black), '68 Coupe, '11 GT/CS, 08 GT Vert