It's not overheat (lol on UT99) or whatsoever. It has something to do with your GPU or Memory. If the BSOD still pops up, post the dump files again.

And it has nothing to do with your OpenGL / D3D. You should use D3D over 100 Hz for smooth gameplay. 72 Hz or lower -> OpenGL, otherwise you will have mouse input lag (can be fixed, but it will be better with GL) and stutter.