I had that problem. I would be swinging in a totally open, grass yard and it would just shut down. I'd start it back up and swing for a while, then it would do it again. There was nothing to bum into to jar the machine, so it wasnt from impacting something. Coulda swore I had charged my RNB battery, so I went home and charged it up and took a AA battery pack with me as back up. Same thing; it would shut down after swinging for a bit so I switched battery packs and same thing happened. Sent it to Kellyco when they did Minelab repairs, and they couldnt find anything. Happened again to me and I sent it back in and this time they replaced the motherboard. Havent had the problem since.
My friend in Seattle is having the same problem right now. He's sent his in twice to the repair center in Alum Bank, PA and they have not figured it out yet, but I'm guessing it will be the same problem. He talked to the tech and all they did the first time was turn the machine on in the morning and set it in a corner and check it later in the afternoon to see it was still turned on! lol Not very conducive to replicating the problem when you're not swinging the coil like you would with actual usage.