Msi R9 390 Screen freezing problem !

Hello;

I use 4 r9 390 gpu with 2x 700w cooler master psu. After 2-3 hours mining, system crashing and screen freeze. I am always restarting pc and open start file many times during a day. Do you know any constant solution? Also PC can't find 4th gpu many times. My driver is crimson 16.9.2 which use for all rx 480 gpu's and never face any big problem like that. I didn't make any OC and flash. Do you have any advice for me?

Thanks for your support.