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You need to pull a brisket when it is probe tender, not when it reaches a certain internal temp. Briskets are done as soon as they are tender, and IT does not determine when that occurs. This has nothing to do with temperature fluctuations, you are most likely just over cooking these. You can use IT as a rough indication of when it's getting close. I start to check around 190, and usually in my pellet grill they are done between 193 and 198.