[Tom Goldman]

Cleaning the weld area is definatly the first place I'd suspect, But as was posted earlier, I've had voltage issues from time to time as I'm at the end of a long run from the high voltage transformer.
Try having a helper moniter the line voltage while running a weld bead, sometimes the volt drop is not evident until the welder is under load.
Also once had a similar problem that resulted from a faulty ground cable that was broken at a factory splice in the cable and was under the insulation and was not visable, Altho I don't suspect you would have that problem with both welders.