2011 wrx 2600rpm stumble issues

Has anyone experienced the 2600rpm stumble at low load cruise speed more noticeable in low gears, 1-3.

I've managed to play with timing from 33 degrees all the way up to 45 degrees of timing, no knock, and changed intake cam advance from 0-25 degrees in the troubles area with no change in the stumble. Air fuel ratio target is 14.7 at this time and i noticed it will bounce when this happens from 14.4 to 15.7. I've re mapped the MAF sensor so the fuel corrections are within 4%.

I'm curious to the "per fuel injector composition tables" are messing with the fueling at this rpm range due to the original design for these fuel rails. I noticed the stock maps have a variance of -3% to +5% at the point of stumble. Also noticing this is when the fuel pump is at 33%. is it possible since the fuel pump is at the lower fuel rate and at cruising speed the variance in ipw per cylinder is causing a stumble to happen by skewing the fuel flow to the injectors. letting one cylinder go rich and the other lean, where it could be possible to tune this out by increasing the fueling percentage and adjusting the ipw per cylinder to keep the rich lean cylinders from happening?

Using the Cobb Accesstuner software, engine is a built EJ235 DeStroker, ej207 crank, ej257 block, manley turbo tuff pistons 8.5-1 comp, Stage 2 brian crower 272 cams, wrx heads with gsc valves and springs, two step colder plugs, TD06-20gh turbo with large FMIC, Cattless downpipe, TGV delete, secondary air pump delete, and injen cold air intake. Fuel system is completely stock

This is a pretty common issue due apparently to resonance in the intake manifold. I haven't had to deal with this before however there's a huge amount of information floating around about it. I'd suggest you start here: https://www.iwsti.com/forums/ecu-tuning-and-performance-electronics/203089-final-tuning-solution-gr-stumble-2400-3000-rpm-using-ap.html

Two methods to fix: software fix by tuning manifold pressure based load compensation, or hardware fix by fuel system change. See Cobb article

https://www.cobbtuning.com/2008-sti-fuel-pressure-regulator-kit-update-technical-overview/