2002 Subaru Legacy B4 Injectors or wiring

****UPDATE*******

We are pulling P0340 error code which is Camshaft Position Sensor Circuit A problem. However, we replaced the sensor found on the timing belt cover and it showed the same error. Then my friend took his sensor (we have exactly the same car) and put it on mine and still shows the same error code. We then took the one from my car and put it on his car and surprisingly that sensor we think is dead works on my friend's car. My friend's auto electrician suggested that the ECU could be getting bad (we couldnt do a test on that claim)

Is it the circuit or it is something else. I called my mechanic and told him the results of the investigation and he says if it continually pops the same error code it is most likely the VVT system (something like that).

Could it be that we were replacing the wrong sensor?

Whenever I start the car it stumbles hard and the CEL will flash (warm or cold) then after about 10-15 seconds it will catch it self and idle fine, free rev fine etc. Then once I put the car in "D" and go to pull off it starts stumbling again and the CEL starts flashing again. If I keep it about 15%-20% throttle it will eventually clear out and drive fine for the most part. After this little procedure the car will drive fine until I turn on the A/C or lights or wipers, then the car will stumble bad when I come to a stop and/or try to pull off. The stumbling issue is worsened considerably by having the A/C on.

I have put a code reader on the car and the only code I was getting was P0340 Cam Sensor Bank 1. I then proceeded to change the cam sensor next to the oil fill (is there another one?) And that didn't solve the problem. Checked the codes again, same code.
I have no idea on what else it could be, once the car is driving it boosts fine, drives fine etc but getting bad fuel mileage