I have had this exact problem happen to me. Hopper was empty, opened a new bag of Hickory Cookin pellets and dumped in, I primed the anger got the Gridiron running and then the temp started dropping. Trying to figure out what was wrong I dug out all the pellets, turns out a few extra long pellets wouldn't let the pellets drop down to the anger right. I broke them up put the pellets back in fired it up, fixed. Never had problem again but check pellets before I dump in hopper.
Cookin pellets is my favorite brand of pellets,I don't know why I got a few long ones and don't care. The problem got fixed and I still use Cookin pellets, great pellets