electricity questions?

I could be wrong, I don't think there is any such thing as 30A 120V service (except for special stuff like RV's).

I'd be for doing a couple of things.

You are saying you have a 120V outlet for your welder with a 20A breaker, and that pops?

First, it could be a bad breaker.

Second, there could be something wrong with the welder. what does the welder say it draws?

Does the welder use a "standard" 120V plug? A 20A rated 120V plug should have the one blade "offset," like this:

Does/ did your welder have that type plug?