I guess that leaves 1 and 7. But hey maybe it fixed itself.
Uhm also...what are you doing at 6-8k in 5TH?! You're right, a fuel pump would show symptoms in any gear, but it would also start to break up at lower rpm as it hears up or seize altogether. But I'm curious how you're testing this.

IMO clogged cat is the most likely scenario. Not sure if that was already suggested.

 

Actually, you should reread exactly what I 1st recommended (and others in this thread have even highlighted and quoted this for you)
And then you call my advice "ambiguous" when there was nothing ambiguous about it.

Then you thank me for my advice, yet clearly you didn't follow it even after the fact. You wouldn't be in this situation if you acknowledged my 1st post. Plain and simple.
People often ignore sound advice on these forums, and it gets frustrating. Therefore, I apologize for sounding "premadonna", but if you want advice in the future, you would be wise to heed what's given and not pay lip service to it.

Good luck with your issues

 

Ok, this is my last attempt.

Did you physically inspect the catalytic converter?
There are 6 fasteners holding it on.
It can semi-easily be removed and inspected.
They are one of, if not the biggest weak points of the 8.
Without knowing for sure, I'd say 80% of all Renesis failures result from failed cats.

They fail from faulty ignition components not burning the gasoline, which in turn gets ignited in the exhaust, clogging, melting, or causing the strata to break up, as did mine.



Note the oxygen sensor.
It was still running decently even with the front half nearly completely empty.
Do you see the egg shaped piece of strata?
Many times the complete honeycomb will break up like that and bounce around inside.
They can lodge sideways blocking the exhaust making the car run bad, and then they can dislodge and the car will run better.
Eventually they can end up in the muffler, requiring a totally new catback exhaust, in addition to a new cat or midpipe.

The codes don't always pop up within a short time, it could take days.
Flashing CELs are ALWAYS misfires.
Initially, it's usually bad coils,
Neglecting them can result in the cat being damaged.

Replacing them can result in better performance, but the damage to the cat has already been done.
Cracks can develop, allowing exhaust to pass through, but eventually the strata breaks down, clogging the back half or catback, gradually impacting performance to the point where you get flashing CELs, misfires caused solely from the cat even with good ignition components.

We've seen it hundreds of times here, it's why it's been mentioned umpteen times in this thread alone.

MISFIRES KILL CATS, BAD CATS KILL ENGINES.

Drop it and inspect it to eliminate that possibility.
If it's bad, gut it or replace it.
If it's not, then continue the troubleshooting.

If you don't drop it and inspect it, then quit asking what could be wrong.