Are you opening the door? I read somewhere that opening the door can add as much as 15 min. to re-coop the lost heat in an electric. That seems excessive but I might buy 8 to 10 mins. So lets say you are cooking ribs using the 3 2 1 method. If you only open the door once and hour during the cook, you have cost yourself an additional hour on your cooking time for the box to have to recoop its lost heat.
I realize since there is no probe involved this might not have been the best example, but it is still accurate, but no sense jumping out there discussing what a 18 hour butt would do.
Electrics are very precise on the heat control but very slow on recovery, mush the same as a electric stove vice a gas.
So if you are a peeper a moper or a spritzer you need a fireburner or something to occupy your time with.
That all being said, I am assuming its an MES and they are notorious for inaccurate thermostats. Get a thermometer, verify its true using boiling water on the stove then test yout box temp and compare the two. if in fact its your box talk to MES. They have to my knowledge, been very helpful and courteous with customers experiencing difficulties.