P0519 issue with Idling on 04 Subaru Forester

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In Short, I have a P0519 Code that is thrown and I can’t figure out the cause.

About my car:
2004 Subaru Forester X Manual Transmission (non-turbo)
202000 miles on the Car
Purchased in 2016, with 150,000 mi
Replaced Engine with an Imported 2.0L at 152,000 miles in 2018
Ran with broken Catalytic converters from purchase until end of 2023
Replaced Transmission in 2022

When I purchased the car the Catalytic converters were shot. CEL light was on from that and threw a P0519 as well. I assumed the bad Cat caused both of them to happen. The symptoms below have happened ever since I owned the car. And before and after the Cat was replaced.

Symptoms I experience:

When shifting into neutral at any time after an aggressive acceleration, the car would sputter. meaning the RPMs dropped so fast that the car would go into too low of an idle. Either the car would then die from low and rough idle, or it would recover to a normal idle after about 5 seconds. I learned to tap the gas slightly to recover the idle when it dipped. This usually happens when taking the off ramp on a freeway, but not every time. This does seem to be temperature dependent. In the summer it happens more often than not, but in the cooler months it would rarely happen, and rather tend to have a slightly high idle when coming off the freeway.

When the car is started cold it will have a slightly high idle. When I get to speed and then go into neutral it will usually stay at the highest rpm it was at while in gear (2000-3000 RPM) and slightly climb until I engage the clutch slightly. This happens for the first 3 minutes of driving on a cold start on a cool morning (<60F). On a warmer day, it will not do this from a cold start.

When idling after it is warmed up, it will shift slightly every 5 seconds. It's not enough to move the RPM needle (most of the time) but you can usually hear a difference in the sound of the engine. I briefly looked at it on the Torque app and it varied between 650 and 700 RPM.

I have tried:
Cleaning the stock throttle body.
Cleaning all the sensors and electrical connections to the throttle body
Idle Air Control Motor
MAP sensor
Throttle position sensor
Checking for vacuum leaks
Checking wiring from ECU to IAC motor (all wires seem to have continuity and have their respective Voltages when the key is on)
Checked and Cleaned EGR Valve
Changing out parts (see below)


Parts replaced to try and fix the issue

New:
Idle air control motor
Throttle Body Gasket
Neutral Switch on the transmission

Used:
Throttle Body
Idle Air control motor
Air intake sensor
Throttle Position Sensor

Other Ideas
Replacing the ECU and MAP sensor
Could it be a vacuum leak somewhere that I already checked?

I am at a loss for anything else since I have tried to replace all of the obvious things. Please let me know what you think it could be! Thanks!
 

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