BUG #5238: frequent signal 11 segfaults

The following bug has been logged online:

Bug reference: 5238
Logged by: Daniel Nagy
Email address: nagy(dot)daniel(at)telekom(dot)hu
PostgreSQL version: 8.4.1
Operating system: Debian Lenny 5.0.3 x86_64. Kernel: 2.6.31.6-grsec
Description: frequent signal 11 segfaults
Details:

I got postgres segfaults several times a day.
Postgres log:
Dec 9 21:15:07 goldbolt postgres[4515]: [292-1] user=,db= LOG: 00000:
server process (PID 8354) was terminated by signal 11: Segmentation fault
Dec 9 21:15:07 goldbolt postgres[4515]: [292-2] user=,db= LOCATION:
LogChildExit, postmaster.c:2725
Dec 9 21:15:07 goldbolt postgres[4515]: [293-1] user=,db= LOG: 00000:
terminating any other active server processes
Dec 9 21:15:07 goldbolt postgres[4515]: [293-2] user=,db= LOCATION:
HandleChildCrash, postmaster.c:2552

dmesg output:
postmaster[8354]: segfault at 7fbfbde42ee2 ip 00000000004534d0 sp
00007fff4b220f90 error 4 in postgres[400000+446000]
grsec: Segmentation fault occurred at 00007fbfbde42ee2 in
/usr/local/postgres-8.4.1/bin/postgres[postmaster:8354] uid/euid:111/111
gid/egid:114/114, parent
/usr/local/postgres-8.4.1/bin/postgres[postmaster:4515] uid/euid:111/111
gid/egid:114/114

Notes:
- Postgres was built from source with --enable-thread-safety
- Tried several kernels, no luck
- I have psql on a different hardware, same problem happens there too
- There are no sign of HW (memory, disk) errors
- No other daemons (apache, nginx) segfault, only postgres

The binaries are not stripped, how can I help finding the cause?

Thanks a lot,

Daniel