Possible valve float

[Wsmccoy]

So I have an 07 with a Z3 turbo, stock heads. So it just did it today for the first time but at 5500 RMP in first gear it kinda sounded as if I was hitting my rev limiter but it shouldn't hit my rev limiter until 6000, it only did it in first gear and it made me throw a code P0300 for misfire. Is this valve float because of the stock heads?

[dsmuts]

I wouldnt think so on stock valve train but I could be wrong. How old are the plugs?