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Does the engine run as smooth as it normally would?
I would perform a compression test.
It could be an intermittent injector.
Compression test may be ok, I would perform a leak down test as well.
Is cylinder 2 lower than the rest?
I've had one engine of 170 on 1-3, 160 on 4, which logged P0304.
It had 25% leakage at TDC, out the tailpipe.
Had a collet jump out of the valve retainer.
If yours only occurs once up to operating temp, I've had one that had a loose valve guide in one head.
I would swap injector 2 with 4, and reaccess.
If it becomes P0304, then have all injectors ultrasonically cleaned.
Have you had any success with your car?
I hope my advice has been of assistance to you.
That's good info, sounds like you may locate it this time.