PostgreSQL crashing

PostgreSQL runs for a while but always eventually (30min - 2hrs) crashes.
I thought it was memory initially but Memtest had no errors after running
for a day.  I don't think it is a hardware issues because I can run other
applications without error.  I have ran both version 8.0.3 and 8.1.1 with
the same error.  I have posted a clipping of the crash from the system
log.  Any help would be greatly appreciated.

Dec 20 17:14:57 server4 kernel: postmaster: page allocation failure.
order:0, mode:0xd0
Dec 20 17:14:57 server4 kernel:  [<c0143271>] __alloc_pages+0x2e1/0x2f7
Dec 20 17:14:57 server4 kernel:  [<c014329f>] __get_free_pages+0x18/0x24
Dec 20 17:14:57 server4 kernel:  [<c0145bfc>] kmem_getpages+0x1c/0xbb
Dec 20 17:14:57 server4 kernel:  [<c014674a>] cache_grow+0xab/0x138
Dec 20 17:14:57 server4 kernel:  [<c014693c>] cache_alloc_refill+0x165/0x19d
Dec 20 17:14:57 server4 kernel:  [<c0146b37>] kmem_cache_alloc+0x51/0x57
Dec 20 17:14:57 server4 kernel:  [<c0142019>] mempool_alloc+0xb2/0x135
Dec 20 17:14:57 server4 kernel:  [<c011fec9>]
autoremove_wake_function+0x0/0x2d
Dec 20 17:14:57 server4 kernel:  [<c011fec9>]
autoremove_wake_function+0x0/0x2d
Dec 20 17:14:57 server4 kernel:  [<c015de0e>] bio_alloc+0x15/0x168
Dec 20 17:14:57 server4 kernel:  [<c026c48f>] sync_page_io+0x25/0xa2
Dec 20 17:14:57 server4 kernel:  [<c026d97c>] write_disk_sb+0x5a/0x86
Dec 20 17:15:01 server4 kernel:  [<c026d9ca>] sync_sbs+0x22/0x2f
Dec 20 17:15:01 server4 kernel:  [<c026da5b>] md_update_sb+0x84/0xc6
Dec 20 17:15:01 server4 kernel:  [<c02706b5>] md_write_start+0x5e/0x8c
Dec 20 17:15:01 server4 kernel:  [<f882faf7>] make_request+0x22a/0x2b3
[raid1]
Dec 20 17:15:01 server4 kernel:  [<c02232e4>]
generic_make_request+0x18e/0x19e
Dec 20 17:15:01 server4 kernel:  [<c02233be>] submit_bio+0xca/0xd2
Dec 20 17:15:01 server4 kernel:  [<c0144812>]
test_set_page_writeback+0xad/0xe1
Dec 20 17:15:01 server4 kernel:  [<c0152ad7>] swap_writepage+0x9a/0xa3
Dec 20 17:15:01 server4 kernel:  [<c01487ed>] pageout+0x8d/0xcc
Dec 20 17:15:01 server4 kernel:  [<c0148a33>] shrink_list+0x207/0x3ed
Dec 20 17:15:01 server4 kernel:  [<c0147cb4>] __pagevec_release+0x15/0x1d
Dec 20 17:15:01 server4 kernel:  [<c0148df6>] shrink_cache+0x1dd/0x34d
Dec 20 17:15:01 server4 kernel:  [<c01494b4>] shrink_zone+0xa7/0xb6
Dec 20 17:15:01 server4 kernel:  [<c014950f>] shrink_caches+0x4c/0x57
Dec 20 17:15:01 server4 kernel:  [<c0149606>] try_to_free_pages+0xc3/0x1a7
Dec 20 17:15:01 server4 kernel:  [<c014318e>] __alloc_pages+0x1fe/0x2f7
Dec 20 17:15:01 server4 kernel:  [<c014329f>] __get_free_pages+0x18/0x24
Dec 20 17:15:01 server4 kernel:  [<c0145bfc>] kmem_getpages+0x1c/0xbb
Dec 20 17:15:01 server4 kernel:  [<c014674a>] cache_grow+0xab/0x138
...