That, or you could look into modifying your existing welder to have more heat settings with a reostat (sp?) or something of that nature. You may have to do some searching, but I'm sure you could find a place on the 'net that could give you directions on how to build one.
That said, you shouldn't have a problem blowing holes on the sheetmetal. You may also have a problem with your existing welder that modifying won't help. Electronics do wear out. If I knew of a way to test your unit (and I'm sure there is a way) to find out for sure, I'd tell you.
I'm only saying this because you might be able to make your current welder servicable, to the point where you could save up some dough to buy a 185 amp model so that you could do some serious welding.