FATAL: Backend cache invalidation initialisation failed

On a Digital dual PII/32MB RAM/8GB SCSI
running the same query 45 times I get:

May 6 09:16:27 digital logger: NOTICE: SIAssignBackendId: discarding tag
2147483646
May 6 09:16:27 digital logger: FATAL 1: Backend cache invalidation
initialization failed
..................
May 6 09:16:40 digital logger: FATAL 1: Backend cache invalidation
initialization failed
May 6 09:17:09 digital PAM_pwdb[397]: (login) session opened for user root
by (uid=0)
May 6 09:17:09 digital PAM_pwdb[397]: ROOT LOGIN ON tty4
May 6 09:21:41 digital logger: NOTICE: LockRelease: find xid, table
corrupted
May 6 09:23:09 digital logger: NOTICE: Message from PostgreSQL backend:
May 6 09:23:09 digital logger: ^IThe Postmaster has informed me that some
other backend died abnormally and possibly corrupted shared memory.
May 6 09:23:09 digital logger: ^II have rolled back the current transaction
and am going to terminate your database system connection and exit.
May 6 09:23:09 digital logger: ^IPlease reconnect to the database system
and repeat your query.

This kills my sistem.
Increasing the memory to 96MB didn't work it out.
In real life I expect more than 45 queries simultaneously.

What can I do ?

TIA
Claudiu