The problem may be in the mismatched software between the two units. The InterLink works best when both units are using the same software level, like both using a 6.XXX version software.
I would first try switching the ports that the 798 and 757 units are connected to just to make sure it is not a bad cable or port on the InterLink itself.
You can try resetting the InterLink itself. This may not be the exact problem you are having but the same rest procedure may help. http://www.humminbird.com/Category/Supp ... deNetwork/
If that does not work, call the Humminbird Customer Service folks: 1-800-633-1468