I wanted to add my experience on my 2008 5EAT Legacy GT
TYPE OF STUTTER/STUMBLE
Steady throttle/Increasing throttle/Both: Steady throttle creates a hesitation/surging effect while accelerating
MODS
None- bone stock
Stock Intercooler
GAS/OIL
93 Octane Speedway Gas
Subaru 5W-30 Synthetic
Ohio (midwest)
MAINTENANCE
Sensors changed:
Changed out OEM front O2 (oxygen) sensor for Denso model. Denso model caused engine to throw P0131 & P0134 codes. Then had dealer change out Denso model O2 sensor for OEM O2 sensor
Plugs changed: No
Coil Packs changed: No
OTHER SYMPTOMS
Check Engine Light: P0131 & P0134 codes on Denso sensor only. Stock (faulty) OEM sensor gave NO codes!
RESULTS
What has and hasn't worked for you:
As mentioned above, I was experiencing a surge/release when holding the accelerator pedal steady between 2500-5000 RPMs. I suspected the O2 sensor was faulty in my car and swapped out the stock sensor (87k miles on it) for a Denso direct-fit replacement from advance auto parts. It should be noted that my stock sensor did not throw any check engine lights or codes. However, once the Denso sensor was in, the car would drive without a code for a while, then stumble/stutter/surge and then immediately throw P0131 & P0134 codes (and subsequently drive fine in open loop mode). I decided at that point to have my dealer take a peek at it. They swapped out the Denso sensor for an OEM O2 sensor. So far no surging and no check-engine lights or codes. Additionally, my dealer also found a vacuum leak near the recirculation valve at/near "a bolt". They previously also replaced my "air intake boot underneath my intake manifold" which was damaged/cracked. The combination of the above seems to have fixed my problem and I am no longer contemplating trading in this car.
If something worked, how long has it been working for: About 24 hours. Will update again as time goes on if anything changes.
Update: Almost 2 weeks later its still running great and no stutter!