I think your problem is very related (or the same) as we are collecting in:
Please go there. Moreover, if you are able to make the problem appear in a more or less deterministic way, please post your hardware configuration (it seems the problem is more likely to happen with laptops) and what you were doing before the problem happened (we are trying to make this happens on developers' computers).
Moreover, follow these instructions of NvFuchs, in order to collect the relevant (?) data for the developers: after the "sad event" happens, reboot, edit the boot options and add a 3 to the options, in order to boot in non-graphic mode. At this point, login in text mode and run nvidia-bug-report.sh, that will collect information in a log file. After that, reboot in normal graphic mode and attach the log to your forum post.
The more information we give, the more easy is for the developers to fix this.