I become mad when similar problem happen to me but not worth to lose nerves because graphic card.
Your card is obvious not good with these temps. On 70-80% fan speed temperature can't pass 80C...
Could be 78-82C depend from situation...
You need to contact store where you bought card, that's fastest solution.
If they not help you, and they need to accept RMA, that's their duty. Where you live?
On my GeForce temperature know to reach 85C with 75-80% fan speed, but when I open side panel she drop for
12-15C, and CPU temp increase for 5-6C and when I open side panel drop as well.
Because of that only solution for such situation is watercooling.
But you have different problem because you say even with open panel temps are still high,
my rapidly drop after 2-3 min.
Did you check over net... maybe someone have similar problem...
I found one guy here, he RMA and new card work on much much lower temperatures...
http://www.tomshardware.co.uk/answers/id-2483070/asus-290x-matrix-platinum-insane-temperatures-load....But very often problem is thermal paste, only you shouldn't open your card if you are not advanced user.
And other people notice higher temperatures but that's AMD design and higher temps are lower than yours.
85C is not disaster if you keep inside closed case and temp stay on 85C...
But can't go up and without side panel should drop.
I will follow to see what happen with your problem.
Good luck, I hope everything will be fine and you will play on nice Matrix.
That's good card, that would be my first target for Radeon or Strix for R9-390X and special 980Ti Matrix.