I bought a new N9 a month ago and it also has this problem. It usually happens when I leave the phone on the table.
The N9 was probably in the store for a long time, because PR1.3 was not preinstalled (installed it after I turned it on for the first time). I haven't reset or reflashed the phone, though. My SIM card is 1.5 months old and worked flawlessly for 2-3 weeks in my previous N9.

I'd also appreciate it if someone could help.