Issues with lagging at 4000 rpm. help

Hi folks,
newbie here.
Have a problem with a 97 v8. She seems to lag hard when the rpms reach 4000. Ive changed wires (plugs), coil pack upgrades, and cannot seem to pin down the trouble. Any suggestions?

Greatly appreciate the help....this is annoying as hejj.
MB

well i had some problems with my 94 with lagging did u run a diagnostic it might be a o2 sensor which are kind of expensive for the t-bird and there are four of them so a diagnostic is definatly needed to tell which one it is i would just take it to auto sone and have the obd2 hooked up to it and it will tell u which bank it is