Just a thought because it happened to me one time...but even though you may be plugged into a 220v outlet, your two 110v sources may be "out of phase".

Now, I'm no electrician...but I learned this the hard way. In other words, you need to make sure the 110v sources are not coming from the same source. One quick way to test this is to take two separate 110v circuits and connect your welder to them.

Where I got into trouble was when I installed a single 220v breaker. Due to the breaker box manufacturer's design, I ended up having to install two 110v breakers and installing them at least two breakers apart.

Again, I'm no electrician so take this for what its worth. If you have a friend who knows this stuff, call him/her and ask them about what I am describing. I scratched my noggin for a couple of hours and then called a good friend of mine and he instantly knew how to fix the problem. I separated the two circuits and it worked like a champ.