Temp increases hours after cook start on 22" WSM

[Mike_S]

While using the minion method I will dial my temps in while starting a cook. I get the meat on, and everything holds steady for quite a while, about 2-3 hours (I try to target 250ish). Then all of a sudden my thermometer (Thermoworks Smoke) will show my temp around 300. That's usually when I panic and start closing vents. After all that happens I can never get it to hold steady again and I find myself adjusting vents for the remainder of the cook. Any idea what may be going on here? My WSM is a few years old now but admittedly I only use it a few times a year, so it may not be as well seasoned as it could be.

 

[timothy]

Going from shade to full sun?
A piece of smoke wood catches fire?
Using a heat sink like water in the pan?
How much meat? A cold mass will keep temps down at the beginning.

 

[Keith Jenkins]

Agree with all of the possibilities Timothy listed above, in particular; did you start with water in the water pan?
If so, and the pan is going dry, that will allow the temperature to rise rapidly when the water is gone.