Brisket takes a very long s l o w time at low heat. I'm talking 4-6 hours at 200 degrees.
You can cheat by boiling the brisket for 3 hours first. Then slap some marinade or what ever you want to use and then slow grill it with tons of hickory or mesquite chips.
Like I said, its cheating but it sure does taste good.
make that 200-225 degrees.
Regarding temp. I never trust the thermometers on grills, smokers, etc... My method of determining the right temp for BBQ is that if I can place my hand on the top of the grill cover/lid/door and hold it there for a good 5 seconds or more before it gets too hot, then the temp is right.