Sierra BIOS problem

[mikmavros]

Thanks for accepting

I recently installed Sierra  and I encounter the following problem ... When I restart or I start up my machine return to BIOS. If anyone knows and can any help be valuable Thank you very much. (When I boot from a Capitan disk in the same machine all  is running well.) Both discs with Clover in Legacy mode.

gigabyte z68x-ud5-b3 F8

I7 2600K

GT 620 ( την βλέπει κανονικά χωρίς Web drivers)

16 GB ddr3 1600