Well, I have to say I've never had this in almost 9 years with my SM066. I never use more than 2 - 3 oz of wood for long smokes, only about one oz for short stuff like burnt ends or fish. I think oldsarge is onto something about hot spots in the firebox. Maybe check clearance between the box bottom and the heater element? I think it should be just clear but not too far away. Also, just my personal taste, but I never use hickory except for smoked nuts. I use peach for pork, alder for salmon, and pecan for beef. Mesquite is out of the question - too harsh.
*Late edit* Just noticed the original post was under fast eddie pellet grills, so maybe some replies are not relevant.