Since you checked all areas that causes to lean, the only thing I could think now is the dirty injectors that can't deliver their normal dose of fuel. Dirty fuel injectors can't flow as much fuel as clean ones, and they can't deliver the correct spray pattern that is so essential for clean, efficient combustion and the horsepower desired. The fuel feedback control system will compensate for the leaning effect once it is in closed loop, but it can't correct the underlying condition that is causing the problem....
check fuel filter too. What is the fuel pressure @ slight throttle? Yeah 18 is playing fire. I know you have a turbo too....very dangerous.