Need advice. I have a 07 350z. Recently engine starts vibrating and a engine code pops up. P0021 exhaust timing I think. I will stop car run car scanner clear the code . Restart car it runs great for about 250 miles then does it again. Engine vibrates I clear code off running good again. I've changed all 4 cam sensors and both vvc solenoids. Please if anyone has advice. Ty