32 Flavours...and then Some!

I pretty much keep my PC running all the time, for good or ill. For the last couple of weeks, at random, I would come into the office, only first thing in the morning, to find it powered off every few days or so. Would power it up, the fans would be a bit noisy for a minute or two, but then she'd quiet down and run just fine for a day or two and I'd come back into the office some morning and find her powered off again. Hit the power button--aside from a temporarily noisy fan, she'd power up into the safe or windows mode window just fine and run just fine through out the day and into the evening...

Started to shut her down safely every night before bed "just in case" to save her from the harsh forced shutdowns. Her fans would be loud at power up for a few minutes, but would quiet to a normal whir within about 2 minutes or so and stay that way throughout the day. Could it be excess dust that was causing her to malfunction? We hadn't blown the dust out of her in quite some time...

We blew all the dust out of her over the weekend--there was a metric fuckton, our bad for not catching that one sooner! Much quieter on power up, and she ran throughout a handful of days, too.

She started making some fan wobbly noises again not long after the dusting out, though; okay, we need to replace a fan. But it's working--I can hear it/see it moving freely and well. Not an utter emergency, but something that needs taking care of...

Cue this morning. It's 0 dark thirty, around 0545, I'm reading emails. PC sounds fine, is acting fine, no "hot" smells as if she might be overheating. All of a sudden--BLAM!! Powers goes in the middle of reading an email. What the everlasting hell???

Internet hivemind--given all the random non-computer saavy person information I've provided here...Any suggestions or ideas what might be wrong with my machine? I'm looking for ideas to give to J who's planning on looking at things A Lot Closer this evening. Thanks in advance for any ideas you might have...