Problems that disappear for no apparent reason will reappear for no apparent reason.

I think this is a core part of the technical mindset/instinct. Any unexpected behavior is suspicious and makes you nervous, not just unexpected bad behavior. Plus it's just a professionalism thing - a problem was solved but I don't know what the solution was, that bothers me. It means I might not be able to fix the same problem next time I see it.

all the time with Windows, drives me up the wall … computers are supposed to be consistent, boringly repetitive, not flakier than a sugared toddler …

I'm a software engineer. You know what's worse than not knowing why your code is broken?

Not knowing how you fixed it.

I’m in tech support; this is a split 25% of my life

I have a vacuum cleaner like that.

The roller brush stopped working, I opened it and prodded the electronic shit inside, still didn't have a clue why it stopped working, put it back together and it works unto this day.

yes, sometimes the quick solution is to disable some module/plugin/extension you don't need. you will never know what actually caused the problem, but when you need it to work urgently, that's acceptable.

All the time. Bonus points for the intermittent problem you thought you fixed and the day shift giving you shit for not actually fixing it.