Engine power under 4k RPM seems low, need maintenance?
 

Beyond the 4k rpm the engine is a beast, but if i shift from 2nd at 4.3k rpm up to 3rd at 3.7k, i feel like ive lost half the power. Its inconsistent too, sometimes running it it seems fine, but other times it seems like power is lower (~half like mentioned before).

Me setup
5th gen, 3.5L VQ35DE
My setup is stock outside of a resonator being removed by the last owner, and the same guy put on a ram air intake.

My diagnostic guesses
I feel like something isnt coordinating right in the engine below 4k. or exhaust isnt as free as it could be?
Intake filter is not in great shape either, covered in part by rust dust.

Questions - Need help
Is this normal?
Is it a vacuum valve? second one opens at 4.5k or something?
Is it exhaust related? CAT blown? (high rpm could power through it?)
Sensors? MAF dirty?
What can i diagnose/clean to smooth out the power?

Better answers found if you post in the 5th gen specific forum.

I figured that this engine and setup are common to more than 5th gen.... but i can see why i would move it, ill post a link to this.