I’m having graphical problems too with Unreal Engine 4.6 and it also seems to impact some Unreal Tournament levels, here’s the link to my post on it, do you have any issues like these as well as this GPU particle problem? If so then I wonder if it’s all related to the rendering or buffering in the viewport, or camera based occlusion or frustum culling:
https://answers.unrealengine.com/questions/137167/unreal-engine-46-preview-graphical-tearing.html