I know what you're are feeling. I lost my trainer follow on while I was still on the buddy box. My instructor, me and a couple othre guys watched is to in like a lawn dart. Right into the creek bed behind our field. It was four months before most of the plane was found. Mine, I'm kind of sure, was be cause one of the gus was showing off his first generation computer transmitter and demonstrating how easy it was to chang channels, and one of his channels was the one I was flying. Don't know for sure, and never will. I finally got back my engine, fuel tank, receiver and three servos, one with stripped gears. A crumpled part of a fuselage, and a ruddrer hanging onto a bent control rod.

The plane suddenly going off on its own, on it's last flight path, is probalby loss of radio contact. Either radio or battery power caused.

Get another. To few good flying days to spend worrying about what happened to the last one, when they could be spent enjoying the one you have. Keep in mind that sooner or later, you are going to loose the current plane. Hopefully it is retrievabale so you can salvage parts. In any case, have a backup ready to fly.. You can enjoy the one flying while you wonder what went wrong with the one that isn't.

Don