For RX 5xx users, May I ask which driver versions are you using?
Since AMD blockchain driver doesn't let me set mclk on one of my cards, i reverted back to using normal drivers, but now the current up to date drivers fluctuate both the core and mclk and drop the hashrate of both cards
The last good drive is still 17.7.1. No point installing new ones, they only break stuff and do nothing good, unless blackchain driver if you know how to overcome the issues.