Try using only one card at a time and see if it still happens. I have a feeling it might be something specific to SLI.
Also, what are you using to monitor the temperature and when are you checking it? Are you using GPU-Z
to monitor the card during
gameplay? If not, do so, and check the MAX temps reached during a game session in it's "Sensors" tab. Be sure the check the box "Continue refreshing this screen..." prior to starting up WoW. 50C seems rather low for these cards...