The operating temperatures are reaching unsafe levels and the card backs off in an attempt to reduce load which reduces temperature.
Keep in mind while you may think '84C isnt that high to be throttling', it's still very hot.
I'm assuming you have an overclock on the card. Try lowering it.
Try adjusting your GPU fan speed profile.
Lastly, if you're using nVidia control panel, change the power management mode to maximum performance:
http://nvidia.custhelp.com/app/answers/detail/a_id/3130/~/setting-%22power-management-mode%22-from-adaptive-to-maximum-performance