You have probably checked this but I will ask anyway. Are all your hose fittings and your flow meter all tight. It does not take much contamination to do what you are describing. If a fitting is even hand tight it will create a vacuum and suck in air from the room and contaminate the gas. Also check along the hose for a nick or a burn spot that might be letting it draw in air. If the cup on the torch is not tight it can pull in air also. If you have too much tungsten sticking out it can do it also. If you like a lot of tungsten stick out put a gas lens on your torch, it straightens the gas flow and allows more stick out before it starts to swirl and draw in air. I am just covering some posibilities of the cause, if I have repeated something allready stated. sorry
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