What temp are you cooking at?
Sounds like your doing everything right up until your pulling it off the cooker...way to early. With brisket (and I only recently figured this out myself thanks in a large part to this forum and of course trial and error) dry and tough mean underdone! Overdone brisket is still moist and tender however it just sorta falls apart. You can't go by internal temp on brisket - you have to go by feel until the meat offers little or no resistance in several spots around the flat. Like butta! You can also tell it's done because the whole piece will jiggle like jello. Once it probes correctly wrap it in plastic wrap and let it hang out in a cooler with foil and a towel for insulation for at least an hour. It can stay this way for hours! Let us know how it goes next time around.
Matt Schneider - "The Smokesman" - Velvet Smoke BBQ & Catering
KCBS Certified BBQ Judge #54767
Weber Smokey Mountain 22 & Superior Smoker SS1
[B][COLOR="Red"]"To BBQ is human, for swine is divine!"[/COLOR][/B]