You need to change to pure argon for stainless. There is a big difference in the metals. I have never done mig with stainless but if it is like tig you have to watch the heat. To much and it will sugar the backside. Not sure if you can set a post flow of gas or not. With tig i have it set for a post flow of at least 5 seconds to keep the weld shielded so it wont sugar. Sugar is basically a bunch of porosity that looks like black sugar. A fab shop i worked at used mig for stainless but on a bigger mig machine that had way more settings then i wanted to know about. Watch the fume plume or smoke. can be toxic. Also stainless takes a litter longer to heat up but not sure how this affects the mig process. Not sure if this helps any.
08 TT FJC
82 Scrambler (Scraps)
81 Scrambler (Sassy)
07.5 Dodge Ram 2500
69 Maro... next project when the Jeeps get done.....