mga with default depth 16 turns computer off???

[Felix Miata]

This 1.13GHz i815 Piii with G400 AGP is multiboot which I've been updating 
over the past couple of days, Fedora 21, openSUSE 12.3, 13.1 & 13.2. It's 
been reliable for years, but tonight it has been acting like the power plug 
has been pulled shortly after KDE finishes initializing my desktop. This 
happened first in 13.2 a few times with 3.11.10 kernel, so I tried and got 
same result in 13.1 with 3.11.6 kernel. Previously xorg.conf was written with 
default depth 24, but tonight I changed it to 16, and only then did the 
crashing start. I've currently booted back to 13.2 but with xorg.conf changed 
back to default depth 24 and it hasn't crashed yet. So it seems like 16 bit X 
with MGA is broken, while 24 is not. Am I missing something? I don't see any 
open bugs that look anything like this, except for one ancient one: 
https://bugs.freedesktop.org/show_bug.cgi?id=8237
-- 
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Felix Miata  ***  http://fm.no-ip.com/