Hi Guys,
              I am new here and I was also having the same issue , till 2 GPUs Z270P was working fine but once I add my 3rd GPU windows was crashing. So I upgrade bios 1st to the latest version of now which is PRIME-Z270-P-ASUS-0610.CAP , For me also Internet download was not working so simply I install from usb stick.

1st : Upgrade BIOS to 610 version
2nd : Go Advanced ----> System Agent (SA) Configuration ----> Enable "Above 4 G Decoding"
3rd : DMI/OPI CONFIGURATION ---> DMI MAX LINK SPEED change to GEN1
4th : PEG PORT CONFIGURATION ---> PCIEX16_1 LINK SPEED change to GEN1

Thats all I did not change PCH CONFIGUTATION configuration like other bro did and for me now mining is stable and running without any windows crash etc. from last 20 hours.


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Kudos
Rebel