It sounds like you're describing that charming behavior called "spitback"...
The usual culprits are, wattage too low, or airflow too tight, or it's been sitting for a while and the wick inside the coil has absorbed a whole lot of ejuice which now goes snap crackle POP when you hit it. Try turning up the wattage a *little*, opening the airflow a little, see if that helps. Usually it will, but sometimes it's just a misbehaving coil, and can only be fixed by replacing the coil.
Andria