Hard to say w/o more info. It's possible it could be resolution artifacting w/ textures. You need to pin it down first. Did you turn off skylight to see if it was the issue? In the past I have been able to turn GI down to 20%. Maybe just play with turning it down. Also try increasing the lighting/shadow quality/accuracy and turn off interpolation. If you don't get anywhere I would report it. Can't hurt.

No need to think outside the box....
    Just make it invisible.