I thought I would post this just in case anyone else is seeing the same problem. I have my MacBook in the shop now to have this fully diagnosed but it seems that I might have a bad power supply. It's intermittent but it snaps off randomly after being on for a while. This happens with either battery or AC power. The problem has appeared over time. I had my MacBook for at least two weeks before I saw this happen. I'll report back when I get a diagnosis but if you see similar behavior, it would be good to know if this is an isolated issue or not.