If you put on a new bottle and think that the problem occurred around the same time, I would put my money on a contaminated bottle. I have encountered contaminated gas bottles several times in my career and it makes for an impossible weld. Very similar to what you described. Unless thoroughly and visibly contaminated, I doubt that a gas lens is the problem. Nor do I believe that all of your choices of tungsten could be bad. I have never encountered a badly manufactured tungsten. I come back to contaminated or insufficient gas.
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Rick
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