I had a little different issue with mine where it would surge at when blade clutch was disengaged and run perfect when blades were engaged. I am not much of a carburetor guy (and maybe a bit lazy to take it apart) so before I did anything to the carb, I ran some seafoam through the gas tank for a tank and after 2 tanks the engine runs perfect at idle, and at high speed with and without the clutch engaged. Not sure what thoughts are on Seafoam, but I run it in my first tank of the season for all my power equipment; Honda or other engines.
The other suggestions of balancing the blade and a fresh plug is just good insurance and maintenance regiment as well.