Don't know your set up exactly , but a few things I would look at/consider might be: (a) waterpan (b)avoid lump (3)don't overstart/overshoot target temp at beginning (4) minimize lid opening (5)check for leaks. Can't think of anything else. I'm not too familiar with webers, having recently got my wsm22. Hope this helps.
RedPig, Swing Dings, KCBS Member, KCBS CBJ, BGE, Peoria Custom Offset, WSM, Weber Kettle, Traeger Lil Tex, UDS x 2, Stoker 2.0