If changing the plugs doesn't help
If swapping the igniters doesn't help
Try this ...
Change your gas station.
I had this issue last year. It was a PITA trying to find the problem. I changed plugs, swapped plugs, swapped igniters, checked igniters, checked igniter connections, changed rear O2 sensor, and a bunch of other items.
In my case, the issue always stayed w/ #2 cylinder.
All of the above * could * be a cause of the P030X code. Lots of searching in NASIOC, IWSTI and other places. I logged every chenage, one by one, to track the issue to eliminate areas of possible trouble.
Finally realized what conditions caused the code.
I used to use the same gas station all the time with no problems. One day, the issue started without warning (don;t they always?), not long after filling up.
Problem would either come and go OR come on for an extended period.
Finally changed to a different gas station and after running a complete tank of fuel on the new stuff, problem went away on its own.
I have not gone back to the old gas station and my problem has not returned.