You might have heard experts say that the stability of an arc has something to do with the size of the electrode in use. Take note of the following points for your references. 1. Too large a tungsten may cause the arc to rotate erratically around the tungsten’s end. 2. Too small a tungsten may cause the current to melt an electrode and result in an erratic welding arc. I know you don't want to allow these conditions to occur. So, you need to match the amperage range to the exact size of tungsten recommended by your seller/ manufacturer. Another common cause of an unstable arc is contaminated tungsten. A tungsten electrode gets contaminated more often than not if your base metal has debris or oxidation from inadequate or impure shielding gas. You can resolve the problem by cleaning the base metal. Moreover, grinding or replacing the tungsten may also help.