First question, are you using straight Argon, or are you using a standard 75/25 Argon/CO2 mix. If your using straight Argon the arc will do exactly what you are describing, alot of popping, etc, and next to no penetration. Maybe at the last exchange the supplier got the mix wrong, had the bottle labeled wrong, etc. Reason I know straight Argon will do this is when MIG welding is when I was working for a body mfg I changed out my tank and grabbed a straight Argon by mistake. The tanks were outside, it was dark, and I simply rolled it to my machine and hooked it up like I usually did at least ever week or so. Anyway, it took three of us nearly 30 minutes of trying to get the thing to weld right, and scratching our heads when nothing we did worked, to ever figure out the actual problem. Hooked the right bottle back up and things were just fine.
I've welded on steel made in alot of different places and, other than the gas mixup, have never had the problems you describe unless it was because contamination of some sort, which is basically what your asking about. Typically when I've had problems it's in a place like a pin bore where lubricant had gotten ingrained in the pores of the metal and leaches out as the metal warms up from welding. I've also had it happen when it was necessary to weld something that couldn't be gotten 100% clean and had a thin layer of rust, flecks of paint, etc, etc left on it. The rust, paint, etc you can plainly see, but the oil can be there even when the metal looks clean as a pin.
Beyond that I don't have a clue. Good luck.