Intel DX58SO2 suspends fine: but only for the first few hours

I have an Intel DX58SO2 based desktop which works great with WOL, and also passes the Suspend/Resume test at
https://wiki.ubuntu.com/KernelTeam/SuspendResumeTesting
But fails if left suspended for somewhere between 4 and 10 hours.
When resumed after this delay the machine hard boots, usually leaving the USB
keyboard dead as well.

# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-2.6.38-8-generic root=UUID=8a67d411-9750-46e4-8a53-b2059e5a4eeb ro quiet splash,acpi_sleep=nonvs

# cat /etc/initramfs-tools/conf.d/resume
RESUME=UUID=86acb365-d5a4-4249-a28c-5ab4b694181b

# uname -a
Linux ubuntu 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:24 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

# dmidecode
SMBIOS 2.5 present.
Processor Information
    Socket Designation: CPU 1
    Manufacturer: Intel(R) Corporation
    Version: Intel(R) Xeon(R) CPU           E5607  @ 2.27GHz
    (ECC Memory is installed but apparently not actually working)

BIOS Information
Vendor: Intel Corp.
Version: SOX5820J.86A.0779.2011.0406.1511
Release Date: 04/06/2011
Characteristics:
ACPI is supported
BIOS Revision: 0.0
Firmware Revision: 0.0

Handle 0x0007, DMI type 2, 20 bytes
Base Board Information
Manufacturer: Intel Corporation
Product Name: DX58SO2
Version: AAG10925-205
Features:
Board is a hosting board
Board is replaceable

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Where can I go next? The builder of this machine claims that Linux suspend/resume has been "been basically a crap-shoot for years", and thus they refuse to even test it out properly.

If I shop for a new motherboard, where other than
https://wiki.ubuntu.com/KernelTeam/SuspendResumeTesting/Feedback
Can I get feedback on which models are most likely to act fine under Suspend/Resume/Wake on Lan?