Relax. It's just fine.
There are three stages to meat temperatures during a long smoke on briskets and butts.
1. Initial stage: rapidly climbing temps. This is physics in action. A cold mass in a hot space will quickly absorb available heat energy in an attempt to equalize with the surrounding environment. It is normal to see the temp climb rapidly until the next stage. The meat temp will usually climb from 37F to 150-160F in 2-4 hours, depending on chamber temp.
2. Stall stage: steady, and occasionally falling temps, for a long time. Chemistry and physics start dancing together at this point. In essence the meat is "sweating" like an overheated muscle, shedding water. The heat also starts changing the meat, beginning to break it down by rendering fat and melting the collagen. An unwrapped hunk of meat can be in the stall stage for 4-6 hours, even longer.
3. Finishing stage: slowly climbing temps. Once the stall has finished, the meat has shed most of the water. The chemistry and physics of the breakdown process takes over. Fat continues to render and the collagen connective tissue starts melting more completely, tenderizing the meat. Depending on chamber temp, meat temp will usually climb 2-10F per hour. The greater the difference in temperature between the meat and the chamber, the faster the meat temp will climb toward completion.
So, all is fine. Relax.
