Bubbles in your paint suggests that there might be contaminants of some kind lingering on your brush. Alternately -- if you shake your paint before using, rather than stirring it -- you might give it a few minutes to gas out before painting.
I have to admit, in nearly six decades of dealing with model paints, the only time I've seen that kind of bubbles were in early-days (and fairly cheap) craft-store acrylics. This has got me kind of stumped.