So I'm in the middle of cutting when I notice a reduction in my mower's rpm, starts to sputter a little, and puffs of black smoke are coming from the muffler. I immediately throttle down, dis-engage, check under the deck & all other visuals I can perform - all is fine. So, I load up and go straight to the local service center. Turned out to be just a fouled spark plug. The mechanic had it solved in 10 minutes. I don't know if any of you have experienced similar problems, but I now have an extra set of plugs in my truck in case I see these symptoms again.