need advice on what could cause this slight engine problem

Sorry for not giving the engine combo setup. i thought it was a simple general question.

This is a new engine build. took a 5.9 mag and stroked to a 408. eddy alum heads, 241/247 duration 750ccfm holley 4160 that has stock primary jets with #80 jets in back instead of meter plate with jet extenders. motor has about 500 miles on it. Reason why i went with #80 jets in secondaries is because when i would try to pull in 3rd my engine would fall on its face at 4000rpm. so i tried to richen up the mixture plus the extenders and it pulls great now.