Try setting all fans to full speed. The GPU temperature reading only checks the core temperature, not the VRMs or RAM. 79C is on the high side. Another gotcha is that if you have "smart" fans, the motherboard fan controller will not know the GPU temperature and therefore will not know to ramp up on high GPU load. (I implemented a workaround on my machine by raising the minimum fan speed if the PSU load is significantly more than idle.)