I don't even open the door until a brisket reaches 185 to 195.
What you are seeing is the 'plateau' like ezgoin says and it will stay in that zone (somewhere between 165 and 175) for HoUrS.
And yes, usually at some point in the plateau, the temp will go down about 5 or more degrees due to the changes taking place in the brisket.
Happy Q'ing!