LWLock acquire - can't wait without a proc structure

Hi all,

I'm running postgresql 7.2.1(linux kernel 2.4.20).
RAID5 4 x 20G, 2G RAM, two Xeon.

/proc/sysctl.conf
    kernel.shmall = 67108864
    kernel.shmmax = 67108864

/opt/pgsql/data/postgresql.conf
    shared_buffers = 4096
    max_lock_per_transaction = 128
    effective_cach_size = 40000

From time to time I receive the message:
"LWLockAcquire - can't wait without a proc structure.

NOTICE:  Message from PostgreSQL backend:
    The Postmaster has informed me that some other backend
    died abnormally and possibly corrupted shared memory.
    I have rolled back the current transaction and am
    going to terminate your database system connection and exit.
    Please reconnect to the database system and repeat your query.".

What can be the reasons of these?

Thanks in advance.