i just experienced the same issue your describing. After buying a new Ethernet Cable, I determined that my issue was the Navionics card. I called Navionics, explained my troubleshooting steps, etc.. Navionics sent me a brand new Platinum South Card and it solved the problem.
One thing I would suggest is make sure the units work fine with Insight pre-loaded maps and you're seeing Waypoints on both units, etc.. If that's working like it should, then it points to a Corrupt Navionics card.
here's my post on BBC on this subject:http://www.bbcboards.net/showthread.php?t=674767
good luck and hope that helps you.