Darn...been trying to figure out this one for the last few hours. I was playing with the paint effects hair and I got some pretty descent results except:

I have a head model and three strokes (+ control and pressure curves) on it which emits the hair. I also have one key light (spot), one fill light (point), and one directional backlight. All of them with dmap shadows enabled. The pfx brushes has also fake shadows set to 3D cast. One of the strokes gives me pretty bad artifacts on the model. If I hide the backlight (and loose it's nice effect) the artifacts are gone. It is only this one stroke with the backlight which maya doesn't seem to like. I've tried to fiddle with the shadow settings, changed the backlight to a spotlight and everything...help!