Like the others suggested, clean/replace tip.
Ensure good grounding.
May try a small 1lb. roll of FCAW-self sheilded wire on mild steel to see how that runs, the rust or cleanliness of the material isn't as important with FCAW-self shielded wire as it is with GMAW(solid wire).
Use the correct size wire for the tip/feed rollers.
Check your extension cord for excessive voltage drop.
John Wright