Last summer I had the same problem. Mine was closer to 15 minutes of run time. At that point in the flight, I would lose one cylinder. I replaced the ignition and the problem was solved. My ignition unit was under the cowl, on top of the motor box. I bought a DA ignition because I was mad at 3W. The DA is a similar design and is compatible with the 3W motor (at least the 3W I bought 5 years ago).

Now today, I have discovered that my 8 month old DA ignition is also bad. I'm going to look for another source. I have had three ignition failures in 5 years.