Step away from the brisket.
Deep, calming breaths
I cook prime briskets at 275 regularly with no problem, and I use Bovine Bold.
What were you cooking in? The WSM? Water in the pan, no pan, something else?
Fat cap up or down?
How did you measure the temp and where in the cooker?
At 275 my briskets go about 45-60 minutes per pound in my FEC-100 as a reference.