Cannot Start Postgres After System Boot

For reasons I do not understand, the Slackware start-up file for postgres
(/etc/rc.d/rc.postgresql) fails to work properly after I reboot the system.
(Reboots normally occur only after a kernel upgrade or with a hardware
failure that crashes the system.)

Trying to restart the system manually (su postgres -c 'postgres -D
/var/lib/pgsql/data &') regardless of the presence of /tmp/.s.PGSQL.5432
and /var/lib/pgsql/postmaster.pid. Here's what I see:

[rshepard(at)salmo ~]$ su postgres -c 'postgres -D /var/lib/pgsql/data &'
Password:
[rshepard(at)salmo ~]$ LOG: could not bind IPv4 socket: Address already in use
HINT: Is another postmaster already running on port 5432? If not, wait a
few seconds and retry.
WARNING: could not create listen socket for "localhost"
FATAL: could not create any TCP/IP sockets

If someone would be kind enough to point out what I'm doing incorrectly
(e.g., removing /tmp/.s.PGSQL.5432 and postmaster.pid when the startup
process complains they're not right) I'll save this information for the next
time. I can also provide the 'start' section of the Slackware init file so I
could learn why it's not working properly.

TIA,

Rich