High RPM loss of power, fluttering noise from ICE

With only 356k miles on my 06, I'm now getting a sudden drop/loss of power at high RPM. At about 80mph on flat land (or about 60mph up hill) if I try to accelerate the power drops off as a fluttering sound starts (not engine knock). The fluttering sounds like 6 inches of celophane tape flapping in the wind at highway speeds.
I just cleaned my K&N air filter, but the plugs, injectors and fuel pump are original. I ran 1 can of injector cleaner (SeaFoam from O'Reilly) with little to no improvement. I'm guessing it's the fuel pump/pressure, since it runs fine at lower RPM, with no loss of mpg or power (still 50mpg average). I know the fuel pump is in the tank ($400 in parts alone). I'll see if I can measure the rail pressure with Torque and my CANBUS scanner.
Any other ideas would be appreciated.
===== dealer diagnosis eventually said clogged catalytic converter =====

 

Oh yes, I have the yellow warning light for the catalyst ratio being wrong (P0240 ?). I believe that is either a bad cat, or one of the o2 sensors. I just ran the Torque scan again, with no other codes showing.
Yes, from a dead stop, if I press the accelerator past about 90% I get the same effect. I can't find a PID for the fuel pressure, but do have the air fuel ratio. I'll look at this on my long uphill run tomorrow (1,000 feet in about 10 miles). I'm guessing it's running lean, since their is no backfires, loud noises like I've seen elsewhere in rich conditions.
Yes, original plugs.
I cleaned the MAF about 50k ago and will repeat. I just replaced HID bulbs, and did not notice any obstructions in the air intake snorkel/box.
Did I mention the original shocks??? I could autocross this thing it rides so stiff.

 

Looking at Torque: 70mph cruise is pulling 120 cfm air intake. Pushing throttle down 90% at loss of power then shows about 290 cfm air intake - so I'm guessing it's not a clogged cat, because it looks like air is flowing.
However, looking at the air fuel ratio - MEASURED under high throttle, this is swinging wildly from about 13:1 to about 16:1 - with a calculated steady 14.7:1. So the ECU is chasing something...
Also, looking at the cat temps, the #1 probe stays steady about 1100F, under 70mph steady the #2 probe is about 850F (but then drops about 30F under 90% throttle - lean ???)
My guess is the plugs or lack of fuel flow.
--- Does anyone have a PID for the fuel pressure, or fuel rail pressure ??????
Torque screen shots are attached: (if you have a Galaxy S3 or S4, you can take a screen shot by swiping the side of your hand across the screen at any time - if this option is turned on in "settings").

 

Spark plugs were replaced at 358,400 miles. They were dry, mostly there, with a thick, light-brown/white crust all over - except the anode tip. PC won't let me upload a photo for some reason.
No improvement in Loss of Power/Fluttering issue. A little less detonation on acceleration.

Recent Observations:
If I stop for 2 minutes, then acceleration is normal, even at 3/4 throttle, up to about 60mph.
But if I'm cruizing at 50mph, then try to accelerate even moderately, I get the loss of power / fluttering noise.
Now, on long uphills, I get a surging every 2 seconds, when close to the losss of power (lack of fuel ???).
Gas mileage plummeted from 50mpg to about 42mpg before the plugs were changed.

I'm going to have Toyota diagnose. I'm guessing low fuel pressure due to clogged filter, or weak fuel pump, or clogged injectors. $400 part (plus labor) for a new tank/filter/pump plus labor will be painfull (if this is needed I'll try for a used one at about $125).
IF ANYONE KNOWS HOW TO MEASURE THE RAIL FUEL PRESSURE, PLEASE LET ME KNOW. I use Torque for CANBUS info, but can't find the formula for fuel pressure.

 

Yes, intake system is open and cleaned MAF. Torque shows 120cfm airflow intake at cruize, then about 290cfm at 3/4 throttle. So I think air is flowing, which would also lead me to the cat being OK, .... I think.

 

So on a whim, I tried PREMIUM gas last night. This helped reduce the fluttering for about 50 miles, and then it came back. Since I'm all over the map with ideas - could the knock sensor be cutting off fuel flow ?
I'm at the dealer now, $150 for just the diagnosis...

 

The dealer says : Clogged Catalytic Converter, please pay $2,200 parts and labor. I'm exploring other options.

 

I found a salvaged cat/pipe, 53k miles, for $600. Had a local shop install for $80.
Old cat was about 50% plugged with tiny black spots covering holes (sorry , no photo).
Cleared the P0420 code, runs great!

Post Mortem, signs that indicate clogging cat:
Engine Check light, P0420 came on about 3 months ago.
Loss of power/sputtering at high RPM/or high load, with this getting worse and worse (i.e. lower RPM limit).

The second 02 sensor was showing swings from about 2.0 to 3.2 to about 4.
Spark plugs replaced, no big improvement.
I tried Sea Foam and Cat-a-Clean, with no significant improvement.

Confusing signals to me:
MAF seemed OK (80cfm cruize, then up to almost 300cfm on high throttle). I think this fooled me as it was short term, since with would take a few seconds for the backpressure to build in the exhaust.