Yes, I guessed it might be overheating as well. I use my laptop on my bed comforter, although I've been using it on here for over a year and have never had any problems until recently (although the lack of storage/possibly viruses might be a cause for that).

I haven't noticed it hard locking or automatically rebooting, it just shuts off. Like I'll go upstairs to eat dinner and leave it on my bed running, usually with Firefox open, and when I come back it's turned off.