Rough edges on Lasso Selection?

Anyone else get these rough edges after airbrushing a selection?

With a cintiq, Painter is almost as good as a real airbrush but these little color blocks that appear down the edges of masks are super frustrating. Its like a crappy chunk of mask in real life, shouldn't digital mask be better? Works fine in Photopaint... Some times they just take color slower than the surrounding pixels so you can overpower the fact that they aren't taking color like the surrounding pixels but on edges like this it really kills the fun.