Originally Posted by gsweave
Todd is correct, lose the extension cord.
Does anything else draw power from this circuit?
Go back to a new 50 amp breaker.
What size is the feed wire? Is it rated for your usage?
Are all connections tight?
This is an interesting combination of both good and bad advice.
The good: go back to a 50 amp breaker and make sure your connections are tight. Check to make sure nothing else is on the circuit which is unlikely to be an issue because the breaker still breaks even with a 100A breaker in place.
The bad: Scott being correct and losing the cord.
A cord can not possibly be causing the guys problem. All a cord can ever do is reduce the available power at the welder which can't cause a breaker to ever break. Undersized feed wire will just act as a fuse(which is very bad) it can't cause the breaker to break.
I'll assume the the Gunslinger is a single phase 220 machine in which case poor connections could be the issue along with some sort of ground fault. The cord however can not be.