I have a mostly unmodified 2004 G35 Coupe. It's been without issue for a while until I changed the spark plugs and valve cover gaskets. Two weeks after that, I tried to bring the car on the highway and the car has trouble revving past 4000 RPM in third gear. The car is less willing to rev as I put it in higher gear and there is a noticeable lack of power in the high end while low end power remains fine. When it reaches around 4000-5000 RPM I head a knocking sound in the engine. The car has no check engine light and no codes either. I've changed both VVT Solenoids and the problem was not fixed. The car revs fine in idle. Any suggestions are appreciated.

 

Are you able to see the fuel injectors while driving? I'm simply guessing at things here, but if the solenoids are working, then maybe fuel or spark isn't. Coils or fuel pressure is what I am thinking to check?

Also, how are your O2's looking? Running lean?

 

Can you reproduce the sound in park? If so make a video, post it on youtube, and put the link here.