I had the exact same issue as you. I have an AMD rx 580 gigabyte aorus 8gb edition. The core clock locks up at max 1365 mhz and the memory clock stays at idle 300mhz, and gpu usage stays at 100% usage after running specific application, and then stopping these applciation. I have not realized this in firefox because I dont use firefox, but in google chrome I did not have this issue. I had this issue specifically issue after running the etherum miner, when i would stop the mining after about 20min I would have this issue (This is not a mining virus I am sure, I ran it with another gpu no problem) (The only way I was able to resolve issue was to restart pc, or just switch gpu mode from graphics to compute or compute to graphics). Another application I realized same issue is Aida 64 gpu stress test, if I stress test gpu for around 30min and stop, then core clock issue comes up. I thought there was a big issues with gpu so I send it to repair, and gigabyte has tested it with their multiple benchmarks and they have not seen any issue relating to core clock so they are returning it to me, it should arrive in my home in Thursday. Can you please test the Aida 64 gpu stress test for around 1 hour with your gpu and tell me if you have this issue when you stop stress test? I have not tested the latest drivers yet because my gpu has been in repair for a long time, but maby it is resolved with new drivers. The last driver I was to use before gpu was in repair was the 18.2.1, now we have the 18.3.2. This issue is very weird. (Just letting you know if you use the amd block chain driver that is beta, issue is resolved. But I dont recommend to use old blockchain driver, especially if you game alot and need stabability). Other then that I am going to contact AMD and see if they can do something about it. So anybody having this issue please try the aida64 gpu stress test for around 1 hour and stop the test and tell me if you are experiencing the core clock and gpu usage stuck at max. Thank you very much if you read the whole thing, there is alot to read!!!!!!!!