79°C on a GPU under load is perfectly normal.
Here's some food for thought.
For comparison my ASUS Rx-580 struggles to reach 60c.. it's a cool card. And that's using default radeon settings.
Are you using Trixx? or Trixx and wattman at the same time? If yes to either then STOP THAT! and start using default settings or Afterburner with factory default Radeon settings (wattman off).
Have you run a memtest? Are you on the latest motherboard BIOS and using approved/known to work memory profiles?
Have you enabled the overlay so you can watch what your gpu temps and fans are doing during the game?
Is it everygame?
What driver version?