in either the render, scene or world tab, there's a slider/thing labled Samples, in the Ambient Occulsion panel. This basically ups the amount of raytrace particles and samples taken from those shadows for the bake. The grit you see is just a low sample/light ray setting being baked to an image. Obviously the higher it is the longer it takes so don't set it to seven thousand six hundred and forty nine, because that'd take a lifetime. Also i'd set the margin to about 5, so you don't get a cut off map. Don't do that if you have a really tight map or a low res image for your image
oh, and rule of thumb, use images with a resolution to the power of 2. Makes them sharper, use less memory and allows them to be called up quicker. Why? Binary, that's why