Ron has a good point as well. The wind can crush temps if its hitting the rig just right. Also, trying to cook at a higher temp like 300 is tough on a cheap offset. Even truer when using a basket to hold the charcoal. Offsets are ALL about airflow. Having a large basket full of charcoal can really hurt the flow of air and make it hard to keep enough heat flowing into the cook chamber to keep a temp above 250.
Have you ever cooked with KBB at the temps you are today? Other than that I have no idea why the Stubbs would die out like you said.
18" OTS, 22" OTG, SJ Silver, Vintage 1981 Coleman Bullet, PBC, UDS, QMaster Senior ATC