As a previous 03 owner and a subaru tuner I'd suggest checking the wiring to the maf or even swapping another maf in. You can also pull the maf and let the car run in SD mode. Don' tbeat on it like this but its good for troubleshooting.
If the car is missing the check engine light should flash btw.
Also @ that milage I'd look into the coils going out or even a bad spark plug.
To anyone interested in this car, you could pick up a junkyard ej22e with sub 100K for 200-300$ and slap the ej205 heads on it and go. leave it on the stock bo and get more miles out of the car.
02/03 ecus are pretty sweet because of the open source carberry rom.