hooper

my 1996 suburban with a 7.4 and a 4l80 in it will only command torque converter lock-up after i hit 70 mph.tranny was rebuilt 1100 miles ago and the tc was replaced. it wll hold indefinately after that or untill i go under 65 mph. i believe that it is supposed to go into lockup after 42-45 mph? my question is , would a custom tune solve my problem, or can anyone with 1st hand experience set me in the right direction to solve my problem? the tranny was rebuilt 1100 miles ago and i drive like i stole it anyway, but i dont want to fork out the $$$ again for burning up the 3rd and 4th clutchpacks in it . i have tried the search button ^, but had no luck. i have no codes and all my sensors are reading a-ok. also i have the stock tire size 245/75 on it and the speedo is only 2 mph off over the whole spectrum 0-95. i have a k&n and descreened the maf, a msd 6al and a blaster 2 coil, indexed the plugs and a flowmaster dual in/out installed. could i have the pcm reading too much airflow for the stock setup and that is causing the problem? i dont recall what speed the tc locked up at before the tranny was rebuilt, so that info is n/a. thank you all for looking and again, any info is greatly appreciated !