Aluminum Puddle Question

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I have an old tig machine, Miller 330A-BP and when i am welding aluminum i get a nice shiney puddle and just when i am about to add some filler the puddle gives a little dance and i hear what sounds like static electricity.  It dosn't occur all the time and i have tried different things incl backing the temp off lightly after the puddle is formed.Most aluminum i weld is 6061, 3003H14 &some 2024 (i think), most times i am using 5356 rod, 9psi straight argon, pure tungsten (green tip'd) electrode; typical welding would be .060 to ,125 some thicker, rare over .250.Again, the puddle forms bright and shiney and gives a little wiggle/jiggle/vibration and makes a sound similiar to the high freq at start.What is making the puddle do that?  Thanks, oj
Reply:Could be "arc rectification".  I've never seen it, only heard it was an issue with older transformer powre supplies.  See the bottom of this link.http://www.welding-robots.com/articles.php?tag=628More on arc rectification at the bottom of this link page.http://www.weldreality.com/tigData.htm