When you say "meat on,what are we talking?
35 lbs of butts,or a couple chicken thighs?
Like wheelz said,empty units,new unseasoned units swing a lot.
I assume you checked/calibrated an independent therm?
I assume you are checking temp at the spot on the cooking rack?
I assume you open the door and dump some heat,if it is spiking?
I assume nothing is touching the temp probe?