I have a Simrad AP 44 and an AP 48 in the flybridge, AC 42 course computer.  I have 2 Furuno chartplotters, TZT 9 and a TZT 14.

Oftentimes, after entering a waypoint, or a "Go To", when I ask the AP in Mode, to navigate, the "NAV" choice is not highlighted, and will not let me chose "NAV".  Sometimes, after turning off all electronics and re-booting them, it will allow me to "NAV".  Sometimes, not.  

A day or two later, it all works just fine.

Driving me a bit nuts as the problem is un-predictable.  Any thoughts.