I weld with 100% CO2 because that is what I have. I am a home brewer so I have several CO2 tanks for my keggerators. I just "borrow" one when welding and it works fine. I don't get bubbles like that. What I get is a little more spatter. The weld still looks good. Nothing that a flap wheel on a angle grinder can't clean up in a single pass.
I would also lean toward a gas issue. Maybe you have a bad tank. I have seen that before. I also use a flow meter not a regulator.
Another tip that I don't know if it applies but throwing this out because I have seen it be an issue that hasn't been mentioned yet and I don't know your welder. With the Hobart welders they ship with flux core wire. Since they ship this way they have the polarity on the welder set for flux core. When you go with solid wire with shielding gas you have to reverse the polarity. If you don't you get crappy welds. Like I said I don't know your welder to say this is the issue but something quick to check.