Thought I'd throw this out there in case anyone's got any ideas - I'm certainly out of them!
My shuttle ST20G5's recently started playing up - it's 3-4 years old, so whilst it's not cutting edge by any stretch of the imagination it's not at the age where I'd expect random components to start failing. Pretty modest spec, athlon 3200+, 80GB HDD, 1GB RAM, and it's not been abused, doesn't run that hot, just does what I need it to.
Well, it's been gradually deteriorating over the last few weeks - started off with a blue screen crash every few days (which tracing on Microsoft's website helpfully made clear it was either a hardware or software fault... thanks Microsoft), then the blue screens became intertwined with random freezes, lock ups and reboots, and it eventually got to the point where it wouldn't even post most of the time (or restarted or froze on starting windows if it ever got that far.)
So, I stripped the thing down, plugged the components back in one at a time and tested it, trying to find the point where it stopped working - and it didn't. After plugging everything in and booting up again, everything worked absolutely fine. Until a few days later that is, when all the problems started again! So I stripped it down but just ran it with the bare essentials this time (just the onboard graphics, hard drive, tried each stick of RAM in turn) and it did exactly the same - worked for a few days before packing up. By worked for a few days, I mean I was stress testing it for a few hours and it was holding up absolutely fine, no quibbles at all. Then all of a sudden it just stopped working again.
I'm thinking it's the motherboard, which is pretty much a dead loss with one of these things (I'll keep it and have a look to see if anything comes on ebay, but I think it's pretty rare since those boards aren't designed to be swapped out.) I'm not sure why it'd work after I stripped it down and re-plugged things in though? That said, if anyone's got any other ideas or would like to point out something that I've missed, I'm all ears!