I feel your pain. Mine started doing the exact same thing during the first year. I called the phone support just a few days after my warranty expired - frustrating since my problem had been happening for a month, but I was travelling. Don't call the phone support. I've spoken to them many times and every time have been frustrated because they don't know what they are doing. They told me my plug socket was bad and I would have to buy a new device.
If you haven't already, take your device into a store. Turns our my cable was bad, so I replaced that. Then, the rep noticed my updates hadn't been added, so they added the updates. Then they had to get a special SIM card (I think that's what it was) so they could repartition my drive because apparently the 1st version of these devices had some disc partitioning issues. After 3 hours in the store, I had a device that worked. Though they never said they knew what this exact problem you are having was. It worked when I left the store, so they called it good.
That was a few days before I left the country to go to Africa for 7 weeks. This was to be my library on my journey as well as my photo album, music library, and document storage (passport and other important scans). 1 week into my trip, all of my data was wiped off my Nook. No trace of anything left. I have no idea what happened. I redownloaded my books and have successfully read them for the past 2 weeks.
Now, today, that same on/off problem you talk about has returned. I noticed online there is another update, so maybe that would solve the problem. Too bad for me that I don't have a powerful enough signal here in Africa to download it.
If you have learned anything since your original post that might assist me, please let me know.
If you havent solved this problem with the NC, can across this thread that might give you some light about this problem: http://bookclubs.barnesandnoble.com/t5/NOOK-Color-General-Discussion/Nook-Color-powering-off/td-p/880030/page/2
I have a friend having similar problems. We are in Africa, so getting a tech to open it is our only solution.