random panic during boot

I put this thread in hardware section because I am suspecting the hardware, but I am open to any other suggestion.

Since I updated to 13.1 from 12.3 I have serious problems booting the system in the morning after one night of rest. I usually need 3 or 4 tries with the kernel crashing during boot or even not loading at all. After spending 10 minutes or more restarting various kernels (desktop, default, debug…) one of them succeeds to boot and goes all the way to install the KDE desktop and run other applications. I can then work the rest of the day with no problem. The machine is a Shuttle XPC about 4 years old, and I never had this problem with former Opensuse releases. It seems that this occurs mainly when the machine restarts from cold.

Here are some elements about the machine:

   Manufacturer: Shuttle Inc
    Product Name: SG31

BIOS Information
Vendor: Phoenix Technologies, LTD
Version: 6.00 PG
Release Date: 07/02/2009
Processor Information
Manufacturer: Intel(R) Core Duo E7400 @ 2.80Ghz
Voltage: 1.2 V
External Clock: 266 MHz
Max Speed: 4000 MHz
Current Speed: 2793 MHz
2 x memory DIMM DDR2 2048 MB = 4GB of memory

orion /root# uname -a
Linux orion 3.11.6-4-default #1 SMP Wed Oct 30 18:04:56 UTC 2013 (e6d4a27) x86_64 x86_64 x86_64 GNU/Linux

I have run the tests with several passes on the whole memory with no error.

And here is an example of end of a message on screen when it fails to boot (but it may even block before with the bios not starting any initialization):

Kernel panic - not syncing: Watchdog detected hard LOCKUP on cpu 1
drm_kms_helper: panic occured, switching back to text console