I had this problem with my Badger 100LG, a gravity feed airbrush. I think it's caused by an accumulation of paint in the tip.
I partly solved the problem by frequent wiping of the tip during spraying, and giving a quick stab of a spray away from the model just before spraying the model proper, especially after a pause in spraying.
I thought the cause could be too thin a paint mixture, or the needle not far forward enough in the paint tip. I readjusted mine and it worked.
This might help - I hope!