My Scan 3XS 1151 has been rock solid for years but now is going in to BIOS on startup to "please enter setup to recover BIOS settings". If I exit BIOS it starts up but feels like something is going wrong:

i7 6700K @ 4.6 (now running at 4000?)
Vega 64
32 Gb Vengeance @ 3000
Corsair RMi 750 Gold

Why does it keep going in to BIOS on startup? is it something to do with overclock settings? Any options to fix it?