PC hangs and reboots from time to time

Question

I have a very strange problem: Since I have my new PC, I have always had problems with it. From time to time the computer freezes for some seconds and suddendly reboots by itself. I've had this problem since Ubuntu 9.10. The same with 10.04 and 10.10. That's why I don't think it's a software failure because the problem persist too long. It doesn't have anything to do with what I'm doing at this time. Sometimes I listen to music, sometimes I only use Firefox, sometimes I'm running 2 or 3 VMs, sometimes I watch DVD. So it's not isolatable. I could freeze once a day or once a week. I put the PC to the vendor twice(!). The first time they changed my power supply but the problem persisted. The second time they told me that they made some heavy performance tests 50 hours long but they didn't find anything. (How can that be that I have daily freezes with normal usage). The vendor didn't check the hard discs because they used their own disc with Windows. (So they never checked the Linux installation).
Yesterday I made some intensive hard disc scans with "SMART" but no errors were found. I ran memtest for 3 times but no errors found. I already had this problem in my old flat, so I doubt that I has something to do with current fluctuation. I already tried another electrical socket and changed to connector strip but the problem persists. At the moment I removed 2 of the RAMs (2x 2GB). In all I have 6GB, 2x2GB and 2x1GB. Could this difference maybe be a problem? Here is a list of my components. I hope that anybody find something I didn't think about yet.

And here a list of my components:

1x AMD Phenom II X4 965 Black Edition, 3,4Ghz, Quad Core, S-AM3, Boxed
2x DDR3-RAM 2048MB, PC3-1333 Mhz, CL9, Kingston ValueRAM
2x DDR3-RAM 1024MB, PC3-1333 Mhz, CL9, Kingston ValueRAM
2x SATA II Seagate Barracuda 7200.12, 1TB 32MB Cache => RAID 1
1x DVD ROM SATA LG DH16NSR, 16x/52x
1x DVD-+R/-+RW SATA LG GH-22NS50
1x Cardreader 18in1
1x PCI-E 2.0 GeForce GTS 250, Retail, 1024MB
1x Power Supply ATX 400 Watt, CHIEFTEC APS-400S, 80 Plus
1x Network card PCI Intel PRO/1000GT 10/100/1000 MBit
1x Mainboard Socket-AM3 ASUS M4A79XTD EVO, ATX

lshw:

    description: Desktop Computer
    product: System Product Name
    vendor: System manufacturer
    version: System Version
    serial: System Serial Number
    width: 64 bits
    capabilities: smbios-2.5 dmi-2.5 vsyscall64 vsyscall32
    configuration: boot=normal chassis=desktop uuid=80E4001E-8C00-002C-AA59-E0CB4EBAC29A
  *-core
       description: Motherboard
       product: M4A79XTD EVO
       vendor: ASUSTeK Computer INC.
       physical id: 0
       version: Rev X.0X
       serial: MT709CK11101196
       slot: To Be Filled By O.E.M.
     *-firmware
          description: BIOS
          vendor: American Megatrends Inc.
          physical id: 0
          version: 0704 (11/25/2009)
          size: 64KiB
          capacity: 960KiB
          capabilities: isa pci pnp apm upgrade shadowing escd cdboot bootselect socketedrom edd int13floppy1200 int13floppy720 int13floppy2880 int5printscreen int9keyboard int14serial int17printer int10video acpi usb ls120boot zipboot biosbootspecification
     *-cpu
          description: CPU
          product: AMD Phenom(tm) II X4 965 Processor
          vendor: Advanced Micro Devices [AMD]
          physical id: 4
          bus info: cpu@0
          version: AMD Phenom(tm) II X4 965 Processor
          serial: To Be Filled By O.E.M.
          slot: AM3
          size: 800MHz
          capacity: 3400MHz
          width: 64 bits
          clock: 200MHz
          capabilities: fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp x86

Answer 1

Now I have another suspicion. I think that either Thunderbird or Firefox freezes my computer, because the last command auditd recorded while a freeze occures was this:

type=SYSCALL msg=audit(1293264555.924:1628298): arch=c000003e syscall=45 success=yes exit=4 a0=4c a1=7fff5b2a297c a2=4 a3=0 items=0 ppid=2376 pid=2380 auid=4294967295 uid=1001 gid=1001 euid=1001 suid=1001 fsuid=1001 egid=1001 sgid=1001 fsgid=1001 tty=(none) ses=4294967295 comm="firefox-bin" exe="/usr/lib/firefox-3.6.13/firefox-bin" key=(null)

and at another freeze one day later it was this command:

type=SYSCALL msg=audit(1293357969.969:10841001): arch=c000003e syscall=202 success=yes exit=1 a0=7f820365a20c a1=85 a2=1 a3=1 items=0 ppid=2663 pid=2667 auid=4294967295 uid=1001 gid=1001 euid=1001 suid=1001 fsuid=1001 egid=1001 sgid=1001 fsgid=1001 tty=(none) ses=4294967295 comm="firefox-bin" exe="/usr/lib/firefox-3.6.13/firefox-bin" key=(null)

Moreover, since 5 days I haven't let run Firefox and/or Thunderbird all the time as I did before. I used Opera instead and I haven't got a freeze since then. So it looks that one of these programs causes the freezes, but I will still monitor it while the next week(s). If I don't get a freeze then, I know it's FF or Thunderbird's fault.

Answer 2

Since the fault is very random, you should run memtest overnight, at least, before trusting the results.