Power loss at 4300 rpm need help please

[Badass_yellowGT]

hey all
i have this dam problem that is driving me nuts i have been working on stangs for years and this is a first here is my problem

when i accelerate hard in 1st,2nd,3rd, i have plenty of power up till 4300 rpm at this point it seems like the clutch is slipping or the engine is dropping off some time because of knock but i don`t hear any. but it will not do it if i accelerate slowy like normal driving .i won`t have the problem it almost feels like hesitation but the power comes back on when i hit 5000 rpm`s. sometimes

i have done a bit of work on my motor but this is got me stumped
these are my engine mods
2002 pi heads and intake only run 93 octane
mac cai
autolight platinum plugs
holley laser shot wires
under drive pullies
accell coil packs
frpp headers with full flowmaster exhaust and high flow h-pipe with
cats

the clutch is stock and looks alright

please any help would be great if anybody has ever experianced this problem or knows of a fix.