[wushaw]

You have a gas issue, your not getting the proper gas coverage on your weld.
Clogged tip, gun not pushed into welder completely, "O" ring leaking...etc


You can get a film of corrosion on your wire and it will not feed correctly and will just quit feeding during a weld and just drive ya crazy, it will feed all day long when not welding and the only way to fix it is to pitch that roll in the trash and start with a new roll.