I have a G37s 6mt with 140k miles and recently there is a problem somewhere in the engine. It happens intermittently, but the engine doesn't make any power above 3k rpm. Once RPM's reach 3k, it slows down relative to my throttle input and very slowly moves up from that point, even when I floor it.. It's almost as if two cylinders just turned off. The car doesn't slow or bog down but I can't accelerate either so I'll be stuck going 40-45 mph until it wants to fix itself. I'll take it out of gear in motion and it will rev just fine all the way to 7k RPM. The problem typically comes and goes after sitting at a red light but it will sometimes drive normally and then the problem sometimes arises when I shift into 2-3rd gear, which is why I just get stuck driving a certain speed. Could be a misfire, but I have no codes. It doesn't feel rough either, it just feels severely underpowered. I have a OBD reader and it doesn't pull any codes. The car is entirely stock except muffler was removed. I have had some starting issues, to the point where I need to jump the car every 3 days if it sits so I'll be buying a new battery.