AO output just blurry noise when rendering large models

I have been using AO in Black & White mode / no shadows and find that on some models it renders perfectly, but on others with identical settings the output is strange–nearly all white, blurry, washed out, pretty much unrecognizable. This error seems to usually happen on larger models–anyone had this issue and (hopefully) know the solution?

Have you tried increasing the “Occlusion Distance” parameter?