Abnormal Exit Quandary

I'm getting this message on Postgres 7.4.1 (Mac OS 10.3)

2004-05-24 15:20:15 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.

This message repeats itself for each back-end that shuts down.

This is the second time in two days I've got this message in my log.
The server has been running fine for about 5 months straight up until
now. The load on the server has increased somewhat in the last few
days, not in the number of back-end processes, but in the number of
SELECTs each back-end performs.

Can someone give me any pointers on what troubleshooting steps I can
take to get my system stable again?