What temp are you cooking at? I've had 2 4# briskets on for 2 hours, at 250* and they are already at 140* internal.
This is how I cook flats.
Start your coals.
While they are coming up to temp, (250*) wipe the brisket, slather it with Worcestershire sauce, then apply rub. When the cooker is up to temp, put the brisket on fat side down. (This lets the fat act as a heat shield, and keeps the meat from drying out.) Smoke for about 3 hrs. to an internal temp of 160* - 165*, then either wrap in foil, or put in a foil pan , fat side up , and add liquid, then seal with foil. (this can be beef broth, or any mixture that you like. I use three Tablespoons. of Steve Raichlins's honey balsamic glaze, (Here is the link)
mixed with 1/4 cup of strong coffee.
If you want you can now finish in a 250* oven, as there will be no more smoke getting into the meat. cook the meat to an internal temp of 190* - 195 *at the thickest part of the flat. When a probe goes into the meat with no resistance the meat is done. Remove from the oven, and let set for at least an hour, without opening the foil. Slice across the grain.
defat the juices and serve, on the side.
2 OTG One red one black
22 1/2" black Bar-B-Kettle with rotisserie ring
Brinkman dual zone charcoal grill
Webber Silver Genises B
3 burner camp stove