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It's possible that the motor is still breaking in.
A compression test will confirm if this is the case.
From the evidence of the spark plugs, if the compression is indeed the culprit, you should get uneven numbers front versus rear.
Another question would be: how is your gas mileage?
Tracking your gas mileage while breaking in the engine can be used as a good indicator of the condition of the motor.
Have you checked the condition of the coils and spark plug wires?
Just as an easy test, try and swap the leads from front set to rear set and see if the conditions change?
Also, check the resistance of the spark plug wires - they should be pretty close to each other.
-Ted
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