can't log in after a certain time

[Nigel Barker <tech@hiroshima-is.ac.jp>]

Hi All,

I'm sure I read about this somewhere, but I can't find it now.

I have set up a few new computers by cloning one of the old ones
(which always work). Also, (and I'm not sure why I decided this was
necessary) I switched the kernel from 686pae to amd64, but still
kernel 3.2

Because of a sound problem, I had to install alsa from source. Those
are the only two changes since cloning the old machine: amd64 instead
of 686, and upgraded alsa.

These new computers are unable to login after a time (20 minutes?
under an hour anyway), but if I reboot them, they work again for a
time.

Where shall I look for the problem?

thanks
Nigel