Da62 (wt g1000) autopilot just shuts off and won't re-engage

Flying the DA 62 on a long haul flight (trying to get my saddle sore achievement) and the autopilot disengages and I can not reengage it It will reengage for about 10 seconds then drop off again.

7500 set altitude, NAV ON, about 2.5 hours into the flight.

I have experienced this before on a previous flight, but do not remember which aircraft/gps system.

Thanks.

Dave