Hey Ed,
Well from what you are briefly describing so far you are on your way actually.
Remember, butt is probably the most forgiving item you can cook therefore your tempurature fluctuations between 220 and 265 are not a huge concern at this point really. I have done butts anywhere from 220 all the way to 320 at different times depending upon how much time i have to cook.
Monitor the meat tempurature until you get closer to the 185 to 195 range (I am, of course, assuming you are pulling not slicing the meat) or keep testing the doneness by pulling on the bone (if bone-in of course, again with the assumptions) and then pull the butt from the smoker and double foil it. Put it in an empty cooler (or a turned-off microwave just to hold the heat a bit) and place some towels on top to hold the heat. Let rest for AT LEAST 1 hour and then pull and serve. You can realistically figure about 1 to 1.5 hours per pound but that is strictly a guideline.
Hope that helps and you will end your issues with the tempurature fluctuations when you move to your Weber!!!!
. I did order a WSM(from this site)
If you have any advice that would be great. 
