My wife's Vivoactive3 has also gone bad regarding the all-day-stress values recently.
Here is what mine (Fenix5) used to look like normally before firmware 7.60, and how it looks now everyday, no change if you are resting, moderately active, sleeping, cooling down after a hard workout, rest days, etc ... doesn't matter, its the same erratic patterns everyday throughout the day.
Yesterday I did a factory reset on my VA3 - image attached.. now it looks far better. Try it
If anyone is having this issue, please call Garmin Support to complain. I am not sure how much they read the forums, so I’m not sure they are seeing how many people are affected by this. When I called Support, they said that the algorithm was FirstBeat technology — they just implement it, so not sure how much they can do about it since they are relying on FirstBeat to figure it out. My husband’s VivoSport does the same thing on ME, but not on him, so all I can figure is that whatever the algorithm is, it does not work well with however my heart works.
I can say it seems to work well for me.
One thing that I've noticed is that any significant amount of alcohol will drive it nuts. I can have maybe one pint of beer a few hours before bed, anything more than that and it effects the stress rating a LOT. If I go out and have a boozey night and get to bed late, the stress will be pretty much red-lining all night until about 5am. If I don't drink any alcohol at all before bed, then I get nice blue lines the whole night.