I had that issue also - when I turned it down - it was fine - I get 99 - perfect. Most times I checked it, however, I did notice that it would state 88 - or something in 80's at times. I would remove my 'meat' thermometer and wait for it to acclimate then test again - 99 fine - however - i am realizing that the thing will drop to even the 70's and it just so happens that when I test it - it is normal.... i think i have slow cooked eggs rather then chicks at this point.
I am testing for a friends, my HovaBator hand turned I have gotten 60 - 80 % hatch rate. This Little Giant is going to be zero - i will look at the thermostat inside and try again -