Out of memory error

Hello,

I first apologize for my poor english.

On my linux box (Suse 6.2 + Postgres 6.5.4/128 Mo RAM) I have a database
with 200 000 records on 7 tables.
It worked fine, but I have got for 2 days this error message :

ERROR:  hash table out of memory. Use -B parameter to increase buffers.

It occurs only with a multiple fields/tables query ; if I remove a condition
in "where" statement, all is OK.
I tried to increase buffer size with BLCKSZ in config.h with no result, and
~> postmaster -B 128 displays

"/usr/lib/pgsql/bin/postmaster does not find the database system.
Expected to find it in the PGDATA directory "var/lib/pgsql/data",
but unable to open file with pathname
"var/lib/pgsql/data/base/template1/pg_class"
No data directory -- can't proceed."

I am going to upgrade to postgres 7.1 next week, but I would like to fix the
problem before, because this database is on a remote server too, and his
owner prefers to keep old version.

Thanks in advance for help