Ruby, Mine was like that the first few times I turned it on to see where it was at before I put eggs in it. I kept scratching my head asking WHY everyone liked this one so much. I got mine set to 99.5 with no eggs for 2 days. I put my eggs in and WHAMO the darn thing would not hold a temp for about 2 days. Seems once you try to adjust it once, look out - even the slightest touch it changes temp a lot. The little red screw you turn is VERY touchy. I only go a hair, not even really noticeable to the eye. I do like mine now and Im only on day 10 of my first hatch.
Glad to hear that you are happy with it. I now have my temp where I want it. Hopefully it will stay there. So far there is alot less variation in temps in this incubator than the one I had to return.