Created attachment 102030 [details]
Xorg.0.log as found on reboot after spontaneous shutdown (openSUSE Factory)

Downstream: https://bugzilla.novell.com/show_bug.cgi?id=856606

I cannot tell whether or not this is distinguishable from Bug 53423

01:00.0 VGA compatible controller: Matrox Electronics Systems Ltd. MGA G400/G450 (rev 04)

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 11
model name      : Intel(R) Pentium(R) III CPU             1133MHz
stepping        : 1
microcode       : 0x1c
cpu MHz         : 1129.685
cache size      : 256 KB
physical id     : 0
siblings        : 1
core id         : 0
cpu cores       : 1
apicid          : 0
initial apicid  : 0
fdiv_bug        : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pse36 mmx fxsr sse
bogomips        : 2259.37
clflush size    : 32
cache_alignment : 32
address sizes   : 36 bits physical, 32 bits virtual
power management:

dmesg preceeding spontaneous shutdown:
[   86.546006] Key type id_legacy registered
[  245.362466] perf interrupt took too long (2506 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
[  745.035804] [drm] Initialized drm 1.1.0 20060810
[  745.081130] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[  745.081142] [drm] No driver support for vblank timestamp query.
[  745.081149] [drm] Initialized mga 3.2.1 20051102 for 0000:01:00.0 on minor 0
[  745.083193] agpgart-intel 0000:00:00.0: AGP 2.0 bridge
[  745.083226] agpgart-intel 0000:00:00.0: putting AGP V2 device into 1x mode
[  745.083254] pci 0000:01:00.0: putting AGP V2 device into 1x mode
[  745.131620] [drm] Initialized card for AGP DMA.

In Rawhide with recent kernels through 3.16.0-0.rc2.git4.1, server 1.15.99.903-2 and xorg-x11-drv-mga-1.6.16, whether or not disabling acceleration, before KDE4 finishes populating the desktop, the PC reboots itself. 

In openSUSE 13.1 (kernel 3.12.22, server 1.15.99.902-307.2, xf86-video-mga-1.6.3-31.9) and Factory (kernel 3.15.1-2, server 1.15.99.903-323.9, xf96-video-mga-1.6.3-35.4) ), no reboot is triggered in KDE3 or IceWM if acceleration is disabled, but if allowed in 16 bit color mode a reboot (13.1) or shutdown (Factory) is triggered upon opening Firefox in IceWM, or soon after initializing a KDE3 session.

In openSUSE 12.3's kernel 3.7.10-1.28, server 1.13.2 and xf86-video-mga-1.6.2-2.5.1, KDE3 and Firefox work normally.

Second system hardware twin except for motherboard and P4 @2.4GHz model 9 stepping 2 (HT) CPU has LMDE, Rawhide, Factory and Cauldron operating normally without any need to disable acceleration or use 24 bit color mode. Swapping G400 cards between the two systems changed nothing.