Remember the days when a car mechanic would actually make repairs instead of just replacing suspect components? As the world gets more complicated, it becomes easier to justify this type of "fix." A hobbyist may actually enjoy the time spent troubleshooting and getting things to work. I, unfortunately (and presumably you) do not get a kick out of these types of annoying "intermittent" problems.
I am responsible for keeping a wide area network of 200 machines up and running. When a machine has a problem, I allow myself 20 minutes to try and repair it. If that doesn't work, I cut my losses and reinstall. Of course, I cheat by using Norton Ghost which means a reinstall takes about 10 minutes + specific configuration.
I would suggest moving your files on to a separate storage device and "blowing up" (reinstall from scratch) your machines - making sure to only install the stuff you need (contrary to what I see as popular belief, a PC cannot have an unlimited amount of software installed on it without eventually going "snakey"). I make it a point of doing this to my home machines at least once a year.
I hope you didn't get hit with the towel I just threw in.
Good luck,
Paul