P0300 & P0174 but drives normal

stealth23

First of all, I apologize that the 300 issues have been all over this forum many times but I just wanted you guys to hear my issue. Im new to Nissan and having a car like this. So please be nice

My 09 370z 6 speed, manual tranny, sports package with Test Pipes
-purchased it a few months ago, drove from Arizona to Texas with no issues and no service Engine soon light(SES).

- After a few days driving commuting to work, SES came on solid but the car didn't hesitate or anything. Drove normal. Code pulled P0300 & P0174.

- So i removed the crappy spectre intakes and installed the K&N typhoon intake, Cleaned MAF(with MAF cleaner), Cleaned Throttlebody, installed new NGK spark plugs and light still comes on solid. Same two codes

-Its been like this for a month or so, SES still on but car drives normal, no hesitation, I can pull all the way to redline with no problem but i noticed the SES starts flashing ONLY between 2500RPM to 3000RPM but still dont feel any problem and goes back to solid SES when below or above those range.

--I check all the vacuum hoses already
--New spark plugs, i didnt gap it, just installed

So is the flashing telling me a misfire on a coilpack ?
can it be the Test Pipes? or maybe a leak ?
should i test the coilpack ?
Low oil ? idk just giving ideas

If theres not much i can do, does anyone know any shops around DFW who knows how to work on this car.
THANKS

PS please let me know if you guys need additional information on my car, I would like to get this issue fixed asap