Welding Tips and Tricks

Hello, I just posted most of what I am asking on my previous post but would like some input. I was having problems with my old 300/ 300 idealarc tig machine, I have welded about a million castings with a miller square wave machine and never had this happen. I never with the miller except on some castings that were Hugh and made odd aluminum that was of a diff compound had to preheat parts maybe 1% did I have to preheat. with the idealarc old machine it would not put down a clean weld would just sit there soot puddle and fall through. I did notice that when the part was warm it would work somewhat better so I tried heating up the part this sheet 1/16 and 3/32 sheet aluminum and walaw perfect cleaning action from the tig super nice stack of dimes welds. what gives?