its the non extreme model, and also no tv connected.
New antec HCG 750watt arriving tomorrow (rebranded seasonic focus plus), so I will report if the issue goes away.
guru3d in one of their reviews actually noted this specific problem can occur with a psu incapable of delivering the voltage.
I believe what is happening some cores are getting turned off when a certain failure condition is detected probably as a failsafe measure. Sort of like how the ps4 pro turns off some cores in games that dont have ps4 pro enhanced modes.
ASRock K6 Fatality Z370
Teamgroup 8 gig 3200 x 4
Samsung 970 EVO
Samsung 850 EVO
WD 3TB RED x 4
Antec HCG Gold 750 Watts
Windows 8.1 Pro
▲ hide details ▲