sudden and frequent drops in hashrates RX480

Hello fellow miners,

i am mining on teamredminer with 3 AMD RX480 gpus, can anyone please tell what is going on here?
usually they deliver 13MH/s each card, but then their hashrates drops all of a sudden to 9-10 Mh/s and stays there for 5 minutes or so, then again and again go up and down similarly.

please someone suggest me a solution, could using HDMI dummy work?

https://preview.redd.it/p45myy8bp4i71.png?width=792&format=png&auto=webp&s=f0c5789649f44be1ac0e2d265c50541093eb07b6