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Have you done a compression test yet on all 6 cylinders?
The first thing you need to do will be to do the compression test so you know the health of the engine itself. You may have a valve that is not sealing off all the way.
The demand for fuel and air is much higher at higher rpms. If you have an intake valve that is not sealing off, you may have that cylinders firing and the pressure back wash into to the intake burning the air and fuel at higher rpms. You have already done all the basics, now it is time to do some testing to see what is going on.
With out doing any testing there is no way of knowing what is going on.
A compression test will tell you alot about what is going on and is the starting point for the diagnostics to find your problem.
You should also do a fuel pressure test.
I do not have a magic ball that will tell me what is wrong. All I can do is guide you through testing so we can find and fix your problem with out spending money throwing parts at it.