Comment # 38 on bug 1028575 from Ismail Donmez

(In reply to Mircea Kitsune from comment #37)
> (In reply to Ismail Donmez from comment #35)
> > I understand your frustration but basically logs show that you are getting
> > random GPU lockups, searching Google for "radeon r7 370 lockup" shows a lot
> > of results even from Windows. So at this point I believe this might be a
> > hardware problem.
> 
> I considered a hardware problem, but that seems unlikely for many reasons:
> The video card is still very new (about an year now) and a solid Gigabyte
> model... I've had another Radeon / Gigabyte card before it, which lasted me
> for nearly a decade. This issue is also triggered only by the KDE desktop,
> when certain desktop effects play or I select some windows (even with
> effects turned off)... I play high-end games on my machine, yet even they
> don't trigger this exact crash! The behavior of the crash also closely
> indicates a software problem... hardware issues usually cause a total freeze
> (Kernel panic), whereas this only freezes the desktop (without mouse
> pointer) then causes the monitor to shut down.

I can't see if in this bug report but did you try to set kwin to use XRender
instead of OpenGL?