I am a new customer with a new install of GTN 750. I'm flying the Aerobask Robin DR401. After inputting what I think is a valid simple flight-plan and beginning to fly, the aircraft arrives at the first waypoint, attempts to turn, but seems to lose the GPS heading and continues past the waypoint in a slightly altered straight (but incorrect) course. I notice that as I look at the aircraft instrument panel, it is no longer in navigation mode. Pressing the NAV button on the autopilot resets and directs the aircraft to the next waypoint, but then the problem reoccurs: the GPS Navigation is disarmed, the waypoint turn is missed, and I must press the NAV button to reset. I shouldn't have to do that at each waypoint. I uninstalled the Reality-XP software and tried again. Same result. Any help would be greatly appreciated.