I think you are peircing and cutting too close to the workpiece. I'm not sure about your plasma cutter size but I have a Thermal Dynamics A80. In the 30 amp setting if I remember correctly the peirce and cut height is .16". At the height you are cutting at you are most likely damaging your tip when you peirce. Molten metal is blowing back into your tip. After that your cut quality will drop and the dross will increase. In thin material like this I can get around 600 peircings before I see a decrease in cut quality.
You should also make sure that you are clamping your ground to the work piece. A poor ground can cause poor cut issues as well.