the engine stopped completely for about 1 second
I suspect you don't mean that you were staring at a stationary prop, or for that matter even lost all power.
A briefly sticking valve (usually due to lead fouling) would seem like quite an abrupt power loss, and is not that unusual.
Other than that, maybe some kind of lean cut; water or dirt in the carburetter, or an overlean mixture due to air leak or carb adjustment.
I'm just guessing.