Do you measure temps with the help of CCC or another utility ?
Make sure the temperature reading is in Celcius, not Fahrenheit.
Idle temp of 75-80C is real high. You must be running your PC in sauna. I have idle temp of 43C when ambient is around 28C. My card is Radeon HD5770.
If this is a new card, take it back.
Otherwise, thermal paste between cooler block and gpu may have gone away. Therefore, you cannot be sure if there is direct contact of gpu with heatsink unless you remove the heatsink.