I have the same problem with my Gigabyte RX580s. And the problem is very easy to replicate, I just need to disconnect LAN cable for few seconds so that claymore disconnects from pool. After reconnecting to pool, frequencies always stay on 300 or 600 mhz. So it's not a power issue, it's a driver bug I think. I don't know if Mr. Claymore is aware of that and if it can be fixed on miners side.

Any of you found solution for this? I tried everything, never had this problem before instaling blockchain driver...