I've only had mine about a week, so it's hard for me to tell if they get dark from me entering the room or from other sources. But, I have seen them 100% stable from time to time also when I peek at the tank.
I'm running my set-up at about 78f. I orginally had it set to about 82f but turned it down when I added the dats. These are the first dats I've ever owned though, so we'll have to wait and see I guess.