A couple serious errors

Over the past couple days I started seeing errors like this in my server logs:

WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back
the current transaction and exit, because another server process
exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and
repeat your command.

Then tonight I started getting one like this:

FATAL: semctl(0, 0, SETVAL, 0) failed: Identifier removed

Postgresql is primarily being used for a web application here, and the
first of these errors starts popping up after things have been running
for 3-4 hours (I don't have any idea about the second because tonight
is the first I've seen it). Once it starts, about 1 in every 3-4 web
requests that hit the database fail.

It may just be coincidence, but this all seemed to start a few nights
ago right after I ran a full vacuum. Since then I've dumped and
reloaded the database and have upgraded from 7.3 -> 7.4.6, but the
problem persists.

I've found that most of the time restarting Apache will cure the
problem, but obviously this is less-than-ideal as a long-term
solution.

Searching google reveals that several other people have also seen
error messages like the first, and that it is not the root of the
problem but a symptom. I'm not sure which logging options to use to
get the necessary details for solving this, but if this isn't enough
to go on, just tell me what to uncomment and/or change I'll get you
more info.

Thanks,
Mike