I agree with everyone else, I would put my money on operator. Even a cheap airbrush will run smooth if you treat her right. Like was mentioned strip it down and give it a good cleaning, dilute your paint for a smoother flow in the gun, and always run a cleaner (i use windex) through the gun after each color. Keep an eye on the tip of the needle as it will "gunk" up. Careful when you clean it. When you are running cleaning solution through after each color, block the tip so the airflows back through the airbrush and out through the cup then run all cleaner out and let until only air is flowing.
You should be painting close to worry free in no time if you do this.
just my
Caution!!! When you do strip it down to clean, do it carefully and remember how to put it back together. Take apart over a clean cloth or paper towel. NEVER take apart over a sink...sometimes little parts like to jump/fall out and go down the drain quickly. Been there/done that