Welding woes...frustrating!!

^^Here's the thing^^ it might not JUST be the extension cord. You could already have drop from the breaker box due to marginal wiring between the breaker box and your outlet. You need really, to start clear back at the breaker box (or have a friend who knows wiring) see what the breaker is feeding it, figure what the wire footage is from the box, and what size wire.

One last scary thought AND THIS IS REAL

Friend of mine years ago had a then-new manufactured home, AKA double wide on a permanent footing. Forgot brand of breaker box installed by manufacturer. Remember this thing is NEW and all electric, AKA electric heat, range, water heater, dryer. One these items, forgotten which, was giving him trouble, and "he smelled" so I went over there. THE CLIPS ON THE BACK OF THE BREAKERS which make contact with the buss in the box were so poor quality, they were loose and arcing and heating and damaging the breakers and the buss!!!!! YOU ARE TALKING HOUSEFIRE, here friends. I was flabbergasted, and remember to this day.

So let's add this what you need:
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Quality of the fuse panel and contact with breakers and buss

Good connections to wire feeding outlet

Not too long wire to outlet and good connections

Proper outlet IE 20A say, and no GFI unless rated for that high current

Proper size and minimal length extension from outlet to welder.
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