You seem to have covered some of the basics. I assume you are using GMAW? or GTAW? If so, are you sure you have the right shielding gas? Could it be contaminated?
Are you using a manifold system for your gas? If yes, could someone have allowed air into the system during bottle changes or maintenance?
Is your regulator and flow meter functioning properly?
Could your gas hose have gotten moisture or oil in it?
Are you getting good gas coverage from your torch or gun?

Are you using a flux-cored wire? If so try another spool from a different lot. (You might want to try that regardless of the process you are using.)

Is your polarity correct if applicable? How about your technique, too much drag or push angle? Holding too long a stick-out?

Is your base metal porous? Is there residual paint or plastic or mold release agent on it (or in it)? Can you degrease the part or heat it?

Without being there to see what is happening I can only guess (not even sure I would be any help if I was there). But... we know something is causing the problem. Either insufficient or improper shielding or contamination of base metal, filler metal, or shielding most likely. If the usual cures don't work I usually suspect some kind of equipment problem.

I wish you luck,
CHGuilford

Sounds like a loss of sheilding gas. Question....are there any fans in the area. With the weather as hot as its been people tend to move fans around. I had a simular problem with porosity and I tried everything to get rid of it. Ended up being a fan blowing away the sheilding gas. It as not really that close either but just at the right angle to effect sheilding.