Although it states that you can't return to an earlier version after you update BIOS to F6 I have come across this kind of situation before.
Although you can't use QFlash or the @BIOS utility to re-flash the BIOS I have managed to roll back to an earlier version of BIOS by creating a DOS bootable USB pendrive, using rufus and then using the EfiFlash tool that is downloaded along with the BIOS files. This would be worth a try if you really want to go back to F5 BIOS.
Another thing to try is going back to the F6 BIOS and see if the problem is present in that BIOS version. You should be able to do this using the QFlash utility.