I'm in my later 40's and I've experienced this phenomenon that you so wonderfully described. I used to call it 'the random' before the word random became a thing. I didn't have any other word for it, maybe precognition or perhaps "thoughts are things" and I too was frustrated because it seemed useless! I don't experience it very often anymore, it peaked in my mid-20's and decreased in frequency over time. I have a few theories about it, most of which are a little woowoo. :P But my theories boil down to... this is a shared phenomenon that many people recognize as being a real and natural thing when it's explained well. Sorry, I'm on mobile and apparently I can't even think well when typing on mobile ha.