Database lost after reboot

Hi. I have PostgreSQL 7.4.8 installed on a SuSE Enterprise Server 9.
When I create a database, everything works fine until I restart the
computer. When I do that, and I try to connect to the database, I get
the following message;

psql: FATAL: database "mydb" does not exist
DETAIL: The database subdirectory "/var/lib/pgsql/data/base/1964658" is
missing.

No dir is removed during the reboot, (I checked that, and the subdirs on
/var/lib/pgsql/data/base were exactly the same before and after the
reboot), and 1964658 does not exist (didn't exist before the reboot,
either), so it seems that postgres loses the reference to the dir where
data is stored. It doesn't happen all the time. The rest of the software
on that computer is working fine.

Does anyone know what can be causing this behaviour?

Thanks in advance