Sounds like porosity to me. Try playing with your gas output on your regulator. How are you welding it, flat, uphill, downhill, overhead? Are you sure your machines settings are correct? I'm not positive about the gas mixture. I am currently working for a company that makes commercial modular buildings. We use carbon steel and weld it with esab 250 mig machines. We use a co2/ar mix and both solid and flux cored wire. The flux core has a tendency to porositize more easily than the solid wire, maybe this is your problem. Also, now that I think about it, are you sure you are using the right polarity?