I have a Rebel which is somewhat like the Assassin and my temps vary like yours but I think it is caused by running the blower. When mine gets up to temp and is there for a while and doesn't have cold meat nearby the temps on the Guru and door are the same. I put the Guru probe on the Door thermometer and they read the same all the way. You will love your cooker. We now have one year cooking on ours in comps and are satisfied with the results. My control probe is just beneath the second shelf about a half inch. I put six sensors in mine, three on each side equally spaced and they vary a lot. Hot on that bottom shelf for sure. You just have to learn where to place the meat you are cooking. I usually run at 265 F.