2002 WRX check engine light problem

[Patsrex]

I took my car to an experienced independent shop after the computer check said it was a cam shaft timing sensor error. The sensor was replaced but did not cure the error. The shop then installed a genuine Subaru sensor at its own expense which also did not fix the issue. I was at 1/4 tank of gas so filled it at a BJ's wholesale station (which I have used for both of by Subarus for years) and the light went out! That lasted one week after numerous starts and stops. Yesterday the car felt like a "hiccup" or "shudder" after stopping at a signal and the check engine light came on and has stayed on. I would appreciate suggestions. The original gas cap was replaced months and I am diligent about tightening it to hear 6-10 clicks. I will try filling the tank with a "name" gas when it gets down to about 1/2. I always use 93 octane as the owner's manual says use 91. No 91 is available nearby.