What are the most common causes of linux system hangs?

["Joe Emenaker" <jemenake@genesiscom.com>]

Aka... "why is my system so well hung?" :)

Every now and then, I'll have a Debian box that starts having fits of hard
system hangs. Sometimes, it goes away when I turn off a daemon. Other times,
it goes away when I put the hard drives in an entirely different computer.

Currently, I'm having this problem with one. Just... out of the blue, it
will hang dead in its tracks. The keyboard doesn't even wake the screen so I
can see if there are any kernel panic messages or anything. Ctrl-Alt-Del
doesn't do anything. I have to hit the rest button.

It doesn't seem to matter what's running becuase I've tried turning almost
all of the daemons (except cron and a couple of others) off, and it still
hung. It's happening more and more frequently, too. It used to be able to go
for a week or two. Now, it barely makes it more than 4 hours or so.

Now, I'm pretty certain that it's some hardware problem. But, I'd like to
avoid moving the whole system to a brand-new machine, find that the problem
has gone away, and conclude that there's just *something* bad about the old
server and that I need to chuck the whole thing.

So, I'd like to isolate the problem, if I could.

With that in mind, does anyone have any personal experience concerning what
the problem usually is in these cases? Motherboard? RAM? Has it ever helped
anyone to *under*clock the CPU?

I'm anxious for any ideas....

- Joe