24 Pin Connector Issue

[bigChungus]

Every time I leave my newly built rig off for a few hours at a time, I have some strange issues rebooting. My PC will hang at the BIOS screen and won't start after a shutdown. The only thing that fixes this is unplugging the 24 pin ATX mobo-PSU cable and plugging it back in. Could this be a CMOS issue, cable issue, or PSU issue? Any help appreciated, just very annoying to deal with. Specs below.

i7 8700k
Gigabyte G1 GTX 1070
ASUS Z370-A II

EVGA Supernova 850 Gold
2 x 8 GB GSkill Trident DDR4 3200 MHz
Corsair H115i liquid cooling
Samsung EVO 512 GB SSD
2TB WD Black HDD