Can anyone explain why I get these dips in GPU Clock speed?

4 x RX 470's
Corsair RM850x PSU
Power cable going to PSU gets very hot - might even start melting if I leave it.

Dips:
https://image.ibb.co/mSP89a/dips.png

Mining:
https://image.ibb.co/fJywvF/mining.png