Max,watch how you go ruining old folks' reps.
The fuller the load ,the more stable it will run.As the good cooks above said,even your home oven may get 35º swings.The therm you dropped inside can read different temps by location.
Yes,it cooks very moist and the bigger the load,the more moisture.Opening it up every couple hrs may lengthen your cook time,but will let out some steam.
Also,as Max says,we sit out here and make wild guesses.After you have tried our suggestions,you will have narrowed it down to what Tony or Bill will ask you next.
As the others said,it sounds like you are doing fine and just need to get used to cooking stuff.