How is the external GPS Receiver connected to the 788ci unit? Direct connection or are you using the Y-cable, InterLink or AS-SysLink cable to connect it to the unit?

If you can: connect up your 788ci unit to someone else’s boat that has a Humminbird unit with an external GPS Receiver. Does not matter that it is not another 788ci unit as all you want to do is test the GPS to determine where the problem is: in the unit or the GPS Receiver (or where it is mounted). This can be done while the test boat is on the trailer. I would make sure that the test boat has the GPS Receiver connected directly to the unit or is using the Y-cable (AS-YC) to connect the GPS Receiver directly to the unit as you do not want to test this with an InterLink connected GPS Receiver.

Also, go into the main menu system and look for the “GPS Override” menus under the “Nav” menu tab. Make sure that it is either set to the “Auto” or “Local External GPS” setting. This should make sure that the unit is not trying to use it’s own internal GPS Receiver.