Its the fuel additive more then likely.I used to use it in my snowmobiles when I stored them for the summer. The problem was it took 3 tanks of gas to wash the stuff out of the tank, until then the thing ran rough. For my old two strokes, it was no big deal but for my new 9 grand, 4 stroke snowmobile, I winced watching the thing idle so roughly.
Now I go out and start my sled about once a month in the summer months. This helps charge the battery and gets the gas to flow through the injectors and keeps things from gumming up. I recommend you do the same thing with your lawn mower in the winter. Start it up a few times...and toss a few moth balls around it to keep out the mice.
By the way, you suck...mowing your grass. Its still snowing here.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"