Odd misfire problem
I've looked through similar issues with p030x codes, but nothing quite matches my recent issue.
I drove my SSEi from Pittsburgh to Boston this past weekend. On the second day of the trip, I at one point opened it up, but around 3000rpm, heard audible knocking and immediately backed off, My CEL came on, the code a p0303, cylinder 3 misfire. I got off at the next exit, filled the tank with gas and put in some fuel system cleaner, and got back on the highway. The code did not return over the next 3 hours of highway driving, though I did not open the car up to full boost for fear that it would.
Once I was home, the code was tripped while the car was idling. It had been tripped prior to the day of driving as well, also at idle. I've noticed no other problems, the gas mileage was great during the trip (30+), and there'* no evidence of rough idle or hesitation. Prior to the first CEL trip, I had opened it up to WOT with no problems, but both times I did after, there was knocking.