arc rectification?

Happy holidays all. Since Mark and I got my machine up to snuff I have been experimenting on and off with aluminum. I have encountered an increasing amount of what I believe is arc rectification as the wave passes the zero current line. I could hear lots of popping and snapping from the arc and covered the flash with my hand to look at the machine and noticed that the popping was the arc extinguishing and the high frequency kicking back in. I'm sure this is a characteristic of this kind of welding but I wanted to, a: confirm that I understand what is happening; annd b: wanted to know how best to deal with it. Is it mostly a shielding issue? I noticed that when I was heating a flat it happened more often but when I went to an edge it almost stopped completely. So maybe my shiellding was bouncing back and causing turbulence in the weld zone? I was using a 1/16 2% ceriated with #7 cup at first... 12cfh. when I switched to a number four cup it seemed to get better too. I also noticed that the tungsten looked a lot shinier after switching to the small cup...