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Sometime around 1993 or so, I was chatting with an auto mechanic about the challenges of fixing things. It's great, he said, when the problem is straightforward with obvious symptoms. The thing about repairing stuff, though, is when the customer comes in and announces that his vehicle makes a weird noise—but only once in a while.

You know, when the symptom is (and here I thought, oh god please don't say it, but he said it anyway) intermittent.

There was a reason I didn't want to hear that word. At the time, I was also a repairman. I ran the service shop of a small computer consulting firm. It's a powerful word for a repairperson of any type to use or to hear. It means you're in for one hell of a ride trying to figure out a problem.

In terms of the car guy's perspective, have you ever had something strange happen to your vehicle while you're driving along? For example, say the dashboard lights suddenly start flashing like a deranged Christmas tree, then all of a sudden they stop and everything's fine. It happens a few times, maybe three or four times in a week. You take the car in and explain what's happening to the mechanic. He's going to scratch his head or stroke his chin or something, and say, "Hmm…so it's intermittent?"

When he says that, he's realizing the problem's not going to happen for him at all. And if it doesn't happen for him, he probably can't fix it. He'll test drive your car for miles; he'll hook it up to a diagnostic computer and run it through half a tank of gas; it won't happen. In fact, the next time it'll happen is when you pull out of the service parking lot and drive home.

It's a similar thing when you fix computers. Someone brings in a PC and says, "It locks up."

You ask, "When?"

"Oh, sometimes. Now and then."

You ask, "What are you doing just before it locks up?"

"Hmm, once I was doing email, once I had left it for a while and came back and it was locked, once I…"

There's no rhyme, reason, or rhythm to the hangs. It just happens. Intermittently.

It could be a heat issue. It might be a bad memory module. Perhaps it's a driver problem. The thing is, there's no way to replicate the problem; you can't cause it, so if it doesn't happen while the poor customer's computer is in the shop, you can't fix it.

Troubleshooting intermittent problems is like trying to find a light switch in a pitch black room the size of a Wal-Mart. You can theorize all you want, you can throw yourself in different directions, but you never really know if you're any closer to turning the lights on until your hand finally finds that little lever. So, say you know someone (perhaps a particular columnist) whose computer occasionally locks up (perhaps it did during a recent podcast recording, and then five minutes after that, and it worked fine for two days, and then it locked up again) but doesn't exhibit any patterns or symptoms?

Pointer Graphic for FingerlinksThink Joel has a problem with intermittent? You should read what he thinks about Windows Easy Transfer.

You could check the logs, such as Windows Vista's Reliability Monitor. In this purely hypothetical case, the logs record nothing more than a "disruptive shutdown." They know the computer has been turned off without exiting Windows, but they don't know that it was locked up before that. No clues there.

You might set the BIOS to defaults, disable all overclocking efforts, check all the fans and invoke a temperature monitor. But you go to get a cup of coffee, come back, and the computer is locked up again with the CPU temp in the normal range.

You might unplug all of your USB peripherals and disable all but the most mandatory background apps through MSCONFIG. Everything might seem fine for a while, you think you might have beaten it, but then you start to type the last part of a column and BOOM, it locks up and you lose everything since the last autosave.

Maybe it has something to do with the user SMASHING HIS HEAD REPEATEDLY INTO THE KEYBOARD. Probably not.

Possibly the most agonizing fact of all is this: You know darn well that this system was working perfectly for about three weeks since its last reformat, and nine months before that. The first time it locked up, the user had done nothing out of the ordinary to precipitate it.

Sadly, intermittent problems can defy even the most thorough troubleshooting techniques and drive even the most experienced repairpeople to the brink of insanity. The trick is to slow down, take a few deep breaths, just relax…maybe go for a short drive to calm the nerves. Hopefully the car won't make that weird noise it's been making…intermittently.

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