[Capricorn]

You might also consider if the HDD in your computer is the culprit: A number of years ago, I had a flaky system for 9 months. I checked everything several times, just like you did, including the HDD for bad blocks, and never found anything. I checked drivers, I looked at the log files of the system and never found anything. I removed any software not really needed. Nothing changed. After nine months, the HDD simply died on me one day. After replacing it, no more flakiness. Simply a stable system again. So, I reckon my HDD was on the cusp of dying, but not really dying for nine months.

And by the way: that was a brand new HDD, that caused the problem. So, it must have had some problems from the start.