PROBLEM: BUG: Constant freezes and kernel panics on a quad core (with dumps)

[Bruno Barberi Gnecco <brunobg@gmail.com>]

Summary: a PC I'm testing is unstable and freezing often. Freezes occur in various 
situations, including under no load (not even the X running). Dumps are provided below.

Sometimes it's a plain freeze, sometimes a kernel panic with the blinking leds and a dump. 
It seems the problem is related to memory, most of the messages are related to null memory 
references (see more below). System ran once for several hours before freezing.

* System specification: Intel Core 2 Quad Q9550 @ 2.83GHz, 4GB DDR3, Motherboard Asus 
P5QC. No overclocking.

* Tested under the following kernels/distros, all had the bug. Since I used their vanilla 
kernels, I'm not posting a copy of the .configs:

- 2.6.27.7-smp (slackware 12.2 hugesmp kernel; distro heavily updated), 32 bits
- 2.6.29.6-smp (slackware 13.0 hugesmp kernel; fresh install), both 32 and 64 bits
- 2.6.29.6-nosmp (slackware 13.0 huge kernel; fresh install), both 32 and 64 bits
- 2.6.31-14-generic SMP (ubuntu 9.10 netbook remix live cd), 32 bits

* Here are some screen dumps (sorry, no serial cable for a text dump).

http://ultraxs.com/share-208E_4B118ED2.html
http://ultraxs.com/share-2FDD_4B118ED2.html
http://ultraxs.com/share-D0D9_4B118ED2.html
http://ultraxs.com/share-498F_4B118ED2.html
http://ultraxs.com/share-0AD8_4B118ED2.html

* Turning off ACPI and APIC, either through the BIOS or through kernel parameters did not 
help, even when only one processor is recognized.

* I ran memtest and no errors were detected. I completed the tests three times.

* Nothing is ever output to the logs.

* A sure way to get a freeze is to recompile the kernel, but the freeze does not happen at 
the same point of the compilation and I can't even say if the compilation triggers the bug 
or if it's just a coincidence and the compilation just takes long enough for it to happen.

* Any other information, testing or debugging you need, just ask. I just can't use the PC, 
it usually freezes within minutes. Please CC any replies to me [brunobg at gmail]. Thank 
you very much for any help.