Well, don't have a good answer, but perhaps your observation about the failure always occurring on the last server

processed (i.e., your server #4) is significant.

Try changing the order of the server processing. Shuffle things around. For example, try making your server #4 the FIRST server processed (i.e., make it server #1), and see if that makes any difference...

Does the problem still occur when processing the same server? If the problem goes away, or if the problem now happens on the old server #3 (which has become the new server #4), perhaps you will want to begin looking at "resource" issues -- instead of "networking" issues...

Just a thought...

-john