Hello,
I also have a 7990.
With GTA I noticed that one of the two Malta cores reached almost 100º and the cooling of the whole system was practically saturated.
To try to correct it, I activated the Overdrive technology in the Catalyst drivers and lowered the 'power limit' to -14%. From there, one core usually stays above 85º and the other at 75º and I can continue playing at a resolution of 6124x1080. You can adjust the overdrive values to your liking, in the end I have left the limit at -11%, with a little overclock.
It all depends on the cooling set you have, but I think modifying that value could help you.
You will let me know, I hope it can be useful for this or other games!
Best regards!