It sounds like you have a wiring problem on your boat, if you're on your sixth unit so far. I seriously doubt that you've gotten six bad units in a row, since statistically, that's be almost impossible.
The problem seems to be the software. I have been told that the "AIS Connection Lost" alarm goes off whenever the unit has been receiving data, but doesn't get anything new for two minutes. That means whenever a ship goes out of range, you get the alarm. If a ship is on the fringe of your reception area, you get the alarm several times.
I sailed from Oahu to Lahaina the other day and got at least 15 false alarms. Raymarine has acknowledged this problem but doesn't have a solution, except to put up with it.