90 amps doesn't sound quite right. Lincoln had several different models in their 135 series. I got the biggest 110/120v I could find at the time. I bought it from Summit Racing. I won't swear to it, but I think I have an older version of this welder here:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=LCN%2DK1873%2D1&N=700+4294925139+4294923770+4294845435+115&autoview=skuUnfortunately, 90% of my belongings are packed away in storage as I'm anticipating a move, so I don't have the slightest idea where my owners manual is offhand.
Anyways, lets assume we're somewhere in the 100-135 amp range.
I paid somewhere around $500 for it.
Anyways, I'll try a 75/25 mix one of these days and see what that does. I just wanted to make sure this wasn't a common problem and easily fixed. I tried reversing the polarity briefly, but that didn't really yield positive results either.
And I did grind the material before welding. I used my angle grinder to bevel the root of the weld, because I wasn't at all certain my machine could handle the thickness.
If the line ends up breaking again, I guess I'll have to go old school and break out the acetylene torches. Haven't done that in a while, but there's no 220 handy that I know of at the store, and they aren't going to let me take the line off property to go to my friend's shop and use a 220 machine.
Mathius