There could be many things going wrong -
Are the specks evenly distributed, do they appear to be IN the paint as opposed to ON the paint? Is your thinner the correct recommended brand for the paint? Did you carefullt dust the model before painting, and try to allow it ti dry in a dust free environment (like under a large tupperware tub)?
Generally, try to paint in an environment without wind or breezes, clean the model with a tack rag (get them from a store that sells hardware or painting supplies), make sure you strain your paint well using fine screens or a piece of nylon panty hose (I assume you're using an airbrush).
Best of all, Kalmach Publishing (see links on the main FSM page) has some really great books on model making in genersl, and airbrushing in particular.
I've got several, and they're really very good and have lots of helpful hints and tips.
Good luck.