Okay Mate.. i just installed my new 7" TFT Touch screen in my car and i am in a good mood so here is my input.....
Your car is hitting R&R.. This is called Rich & Retarded.. This is a mode the ECU goes in when it detects that the engine is getting more air then it would in stock form. So the ECU takes out about 10 to 15deg of timing and adds about 40% extra fuel.
This causes the car to run like absolute shit. Nissan did not want people buying their skylines and turning the boostup, thrashing the f**k out of them and then blowing them up and returning them back to standard and claiming waranty.
Now this has worked, it made it VERY hard to turn the boost up and do mods to the car without changing the ecu, this is why the Apexi Power FC was introduced, to overcome these issues.
Your car has hit a brick wall, there is nothing you can do to fix it untill you buy a aftermarket ECU or use a rubbish bandaid fix like a signal bender (the nissan gods would cast a curse on you for going to the darkside if you do this)
To keep the Nissan gods happy, you should purchase a Power FC and get it tuned and this problem will go away and you will achieve the power curve that i have shown in this pic.
Hope this helps, sorry to say but there is NO way around this appart from the solution I have given you.
PS, PM me if you want to buy a power fc for $700USD
good luck with it.