I think you most likely have a wayward on your hands as you most likely haven’t been married long enough for a walk away. Your gut is right 90% of the time so my guess is she’s cheating with the gentleman you suspect.

She’s getting her needs met with both of you. Connection and uncertainty with him and certainty with you. I would stop the cuddling and playing with her hair (seriously) and don’t be available all the time. Don’t be exciting and fun around her but be miserable and mopey as that’s more attractive (JK). How old are the two of you? Assuming no children?

M:51 W:46
T:22 M:16
S:15 D:11

“Don't chase people. Be yourself, do your own thing and work hard. The right people - the ones who really belong in your life - will come to you and stay.”- Will Smith