Backend often crashing

I have one of those nasty problems, with Postgres backend often crashing
with signal 11.

I'll do my best to give you the details:

Postgres is 7.2.1, more exactly is Debian package 7.2.1-2 from the Stable
(Woody) distribution -- I'm forwarding copy of this message to Debian's
package mantainer.

Postgres is running as a backend for a well known italian web site, running
on Zope (version 2.6.1 with psycopg Python adapter, v.1.1)

The problem is recent, i.e. never happened until last month or so, on this
same setup.
I have a few other machines, running the same software setup, but different
Zope sites, never experiencing any problem.

These are the relevant lines from syslog

Feb 20 14:43:53 speed postgres[13365]: [25] DEBUG: server process (pid
15906) was terminated by signal 11
Feb 20 14:43:53 speed postgres[13365]: [26] DEBUG: terminating any other
active server processes
Feb 20 14:43:53 speed postgres[15908]: [26-1] NOTICE: Message from
PostgreSQL backend:
Feb 20 14:43:53 speed postgres[15908]: [26-2] ^IThe Postmaster has informed
me that some other backend
Feb 20 14:43:53 speed postgres[15908]: [26-3] ^Idied abnormally and
possibly corrupted shared memory.
Feb 20 14:43:53 speed postgres[15908]: [26-4] ^II have rolled back the
current transaction and am
Feb 20 14:43:53 speed postgres[15908]: [26-5] ^Igoing to terminate your
database system connection and exit.
Feb 20 14:43:53 speed postgres[15908]: [26-6] ^IPlease reconnect to the
database system and repeat your query.
Feb 20 14:43:53 speed postgres[15904]: [26-1] NOTICE: Message from
PostgreSQL backend:
Feb 20 14:43:53 speed postgres[15904]: [26-2] ^IThe Postmaster has informed
me that some other backend
Feb 20 14:43:53 speed postgres[15904]: [26-3] ^Idied abnormally and
possibly corrupted shared memory.
Feb 20 14:43:53 speed postgres[15904]: [26-4] ^II have rolled back the
current transaction and am

I immediately thought of an hardware problem but, having an equivalent
machine online, I dumped the db and moved to that.
The problem manifestated at once on the other machine, which had previously
(~1 month before) run the site without any error.

The two machines have the same software setup, but different Linux kernels
(2.4.19 vs 2.4.20, reiserfs vs ext3), and different hardware.

I cannot reproduce the problem reliably, though on the production machine
the database crashes many times an hour.

It _seems_ to be related to some mildly convoluted query (a SELECT only
query). Running that query manually, I managed to crash the backend only
once.
VACUUM FULL never gave any error, nor did pg_dump.

I obtained some (pretty large, ~90MB) core files from the crashes. The
backtrace is consistent between the files, here it is:

#0 0x08157e92 in MemoryContextReset ()
#1 0x08157eb9 in MemoryContextResetChildren ()
#2 0x08157e8b in MemoryContextReset ()
#3 0x08157eb9 in MemoryContextResetChildren ()
#4 0x08157e8b in MemoryContextReset ()
#5 0x080c5c88 in ExecScan ()
#6 0x080cb61a in ExecSeqScan ()
#7 0x080c4139 in ExecProcNode ()
#8 0x080cbe2c in ExecSort ()
#9 0x080c41c9 in ExecProcNode ()
#10 0x080ca630 in ExecMergeJoin ()
#11 0x080c4189 in ExecProcNode ()
#12 0x080cbe2c in ExecSort ()
#13 0x080c41c9 in ExecProcNode ()
#14 0x080cc0ae in ExecUnique ()
#15 0x080c41d9 in ExecProcNode ()
#16 0x080cd5d5 in ExecReScanSetParamPlan ()
#17 0x080c5cac in ExecScan ()
#18 0x080cd5f6 in ExecSubqueryScan ()
#19 0x080c4169 in ExecProcNode ()
#20 0x080c73f8 in ExecProcAppend ()
#21 0x080c4129 in ExecProcNode ()
#22 0x080cbe2c in ExecSort ()
#23 0x080c41c9 in ExecProcNode ()
#24 0x080cb9a6 in ExecSetOp ()
#25 0x080c41e9 in ExecProcNode ()
#26 0x080cbe2c in ExecSort ()
#27 0x080c41c9 in ExecProcNode ()
#28 0x080c30fe in ExecutorEnd ()
#29 0x080c2797 in ExecutorRun ()
#30 0x081104de in ProcessQuery ()
#31 0x0810ed70 in pg_exec_query_string ()
#32 0x0810fd5e in PostgresMain ()
#33 0x080f6d4e in ClosePostmasterPorts ()
#34 0x080f669f in ClosePostmasterPorts ()
#35 0x080f5882 in PostmasterMain ()
#36 0x080f5391 in PostmasterMain ()
#37 0x080d4e18 in main ()
#38 0x401d114f in __libc_start_main () from /lib/libc.so.6

Any hints are welcome.

ciao
Guido