Download the Lr 5.2RC from Adobelabs. There was a bug in exporting which is fixed in this release candidate.
Recently I upgraded to Lightroom 5.0. I have been noticing that when I export images at 1000px max and 100% quality, the image is very grainy. I can open that same image in Photoshop CC from Lightroom, resize the image to 1000px max, and save, and the image looks great. Below is an example. The first photo is the Lightroom 5.0 export. The second is the Photoshop CC resize and save. Does anyone know what the problem is? I never had this issue with any earlier version of Lightroom and I have been using it since version 1.0.