GMCdp,
I had the same problem with my floorpans. I had my MIG set at its lowest voltage and the wire feed speed just right. I would tac here and tac there, but just as I was almost done, burn through. Soooo frustrating! I gave up and had a pro do it. Do you grind off the weld and start over? If so, do you think you thinned the metal just a little each time? I think that is what I did. I was dealing with paper thin metal after trying so many times!
Since then I have had much more luck when I back up the metal with copper. Also, when I go back to a previous tac I aim the wire at the previous bead, then kinda do a "U" to extend the bead. It seems that the initial contact on the previous bead then moving to the metal works for me. I'm far from being a pro, so listen to the guys above.
I feel for ya buddy!
Good luck.
