Surface shaders = death to most computers. Of all the computers I've used in production, none of them liked them; which is strange as everything else is built off of them.

One way you could get around this, it create a lambert, set it's color to zero, and map the out color of the ambient occlusion shader to either it's ambient color, or incandescence. I can't check at the moment, but what it should do, is mimic the surface shader.

Give it a go!