Ya, I have read a lot of those posts too, and I am grasping at straws as I feel there must be a better solution than covering up the symptom of whatever the issue is with higher octane gas.
I am running the 87 Octane, and you are 100% right, when I run the 89% the issue fades. The same with the custom tunes, If I use the 89 custom tune with 91 octane, it works fine, but the 91 octane tunes with 91 make the nice Rattling/Tinny noise noise under heavy load. There is no stuterring at all, under load or at idle sounds smooth, just makes the rattling noise under heavy throttle, and the engine coolant temp jumps with heavy throttle.
I have 0 mods on the car currently. I just paid it off so hopefully that will change when I get this licked.
Car is running stock plugs (Originals actullay, and 30,000 miles on them, was thinking of swapping those too even though they are supposedly good to 45,000).
Wrong A/F - Don't have a wideband, so not sure.
Bad gas... This one bugs me.... I can find no info on how to tell good vs bad gas brands. I generally use Mobil which is supposed to be Top Tier, but I can still find no specific ratings on Ron vs Mon values accross brands to see how it compares.