Again, it's not just about temperature. It's also about how much power the card is drawing. That's why it's called Powertune. The firmware on the GPU will not allow the card to draw more watts than it thinks it can handle. If it starts to draw too many watts, it will throttle the card. The 290X draws ~300W, and you have two of those chips on your card. But your card is only spec'd for 500W. Surely you can see how that is problematic. And you are overclocking, which is only exacerbating the problem.
Drop your clocks down to at least 1000 Mhz, if not 975 Mhz. Then start adjusting Powertune, starting with 0 and decrease it until the card doesn't throttle as much or as often.
Drop your clocks down to at least 1000 Mhz, if not 975 Mhz. Then start adjusting Powertune, starting with 0 and decrease it until the card doesn't throttle as much or as often.