Your PSU should be fine to withstand that GPU. Try remounting it and see if that solves the issue. Also do you get any beeps in result of your system crashing?
Check this and it'll correspond to any error codes you may be getting
http://www.computerhope.com/beep.htm
If not, I would also try booting your system into safe mode and uninstalling your display drivers from Device Manager and the whole Catalyst Control Centre.
Then reinstall your display driver but don't install the beta. I think the current one is 15.7 and then if it stable from then you may want to try the beta driver.
Otherwise check all your connections and such.
Hope this helps