Bug#377386: Workaround

["Evgeny M. Zubok" <evgeny.zubok@tochka.ru>]

I have the same issue. See #370382
and https://bugs.freedesktop.org/show_bug.cgi?id=7112

I discovered that the problem is in acceleration. 
Workaround is to disable acceleration :(

Section "Device"
    Identifier  "s3_trio64v2"
    Driver      "s3" 
    Option     "noaccel"
    ^^^^^^     ^^^^^^^^^
    Option     "DPMS"
EndSection