H3 5.3L stumble under load.

I have a 2009 Hummer H3 Alfa with the LH8 5.3. I had this problem come up over a year ago and i have been working on it off and on. The truck always ran great, i was on a 6 hour trip, got gas and ss soon as i pulled out and got up to speed it started to stumble even under a light load. It has done it ever since.

1) I had good oem plugs the year before this started.

2) Local dealer drove it and it had multiple misfires on all the cylinders. It showed a P0300 code. No pattern to the misfires. They did the top end clean and changed the plugs. Told me the cats were plugged. 

3) i dont believe the cats are plugged. It still has plenty of power, it just stumbles the whole time while at wot. This particular dealership's service has went down hill fast after personnel changes. I still had P0300.

4)I took it to another dealership. I had just started to get another code for the traction control. They replaced my unit and said my stumble was caused by the traction control/4wd selector panel. Said it doesnt stumble when it is disconnected. It drove good for about an hour. No codes and still stumbles when panel is plugged in or not.

On my Xtool D8 I have
1) Good even exhaust temps on both banks.
2) O2s are active on both banks.
3) Fuel trims are in line.
4) It does it after it warms up.
5) Starts and idles perfectly.
6) Temps before and after cats are close for both of them.
7) Exhaust back pressure gauge shows no more than 1 psi in all 4 locations.
I sprayed carb cleaner on the manifold and connected hoses.
9) Cleaned maf.
10) Fuel injector bypass test was good. FP was about 42, test would jump it to 60 then cycle injector til 30 psi remains. All cylinders were identical. 
11) Performed the cylinder load test. When i turned fuel off to injector the engine would drop rpm. All did the same.
12) Performed the fuel pump test through the scan tool. Gauge will show 80 psi when i raised it as much as it would go. I even ran this pressure on test run to see if it would change the stumble. It did not.
13) There is no missing or misfiring through the intake or exhaust. Just the noise of the engine stumbling/surging. 

I think that covers it

What brand is the maf? And is it clean? What codes are present?

Maf is GM. I cleaned it although it looked very clean. No CEL.

On a good working Catalytic Converter, the exhaust temperature should be hotter leaving the Cat, than it is entering.

When checking exhaust back pressure, did you increase the engine speed to a steady 2500 RPM's?

For the exhaust back pressure test I went to about 1950 rpm through the scan tool. That was the max it would do.

I will have to do the temp test on the cats again to check the difference.

Are you still getting a P0300? If so, check OEM data or Mode $06 to check misfire counts. P0300 on this year of GM does NOT mean the miss is truly random or multiple. You'll get a P0300 with a single cylinder misfire. Don't go off of what the dealership said. Use the scan tool and find out for yourself.

If you find that all cylinders have even counts, look for something systemic. If you find one cylinder significantly higher than the others, chase that one first.

There is no codes anymore. The cylinders will show just a few misfires in the single digits.