Smoking in Kettle Grill
A couple of suggestions to help maintain constant heat.


Time it such that you are only adding a little fuel at any one time.  Adding half new fuel will cool down considerably as it has to catch, burn, etc.  Adding 10% fresh at one time is much better, for example.
Conversely, if you're lookin' you ain't cookin'.  Open as little as possible.  Remember there will be a lag to any specific change.  If you change the vent, look at the temp after 5min then after 10min to see if the trend is up or down, and if it is accelerating or slowing.  Use that as a guide to your next change.
Thermal mass is your friend.  Get yourself some appropriate stones, bricks, etc.  If you have any room left over from your fire and meat; fill the space with the bricks.  They will help to smooth out the heat from open/closing as they take up space that would otherwise be taken by "cold" air when you open to refuel.  Obviously you need to not block actual ventilation.


Overall the kettle is a nice grill; but leaves a little to be desired for smoking.  The offset firebox style where you can refuel without disturbing the cook-box has a definite leg-up primarily for the refuelling reason; but with some practice you should still have wonderfully edible results with the kettle.

Happy Eating!