How come nobody ever says what brand of parts they used?

They may be new parts but if they are the wrong parts, you still have a issue. If the new plugs are already Iridiums, then they "should" be okay.

If this car was in my hands, I'd have my scanner on it and watching it while I drive to figure out these problems. It may or may not be misfire related but thats the first thing I'd be watching for.