Here is what I would do.
Download the latest version of GPUz, open the sensor tab and tick the log to file and continue to refresh while GPUz is in the back ground. Bottom left corner.

Now select where you want the log file to be stored and then start playing your game.
Once you get the drop in FPS exit the game and go have a look at the temps on the sensor tab, or if you system crashes then look in the log.
To help us help you - please provide as much information about your system and the problem as possible.