Welcome. What you have going on there sounds like it's four stroking. It is normal for this design to four stroke at cruise, and is especially noticeable until the engine gets a few hundred miles on it.
Have a look at your plug, and if it is really black and oily, then you'll need to make a couple of small changes.
Use a break in mixture of 24:1 at the very lowest! Do not use a 16:1 mixture.