boot manager gone

[xzimppledink]

Dual boot, linux ubuntu and windows 8, m2n-sli main board. I continually experience "missing boot manager" and computer refuses to boot in either OS. Suggestions on the reason wanted. It usually seems to "fix it's self" at a later time but not always. About the time I am ready to smash it it starts working again. I can always hit F8 and start it up.

 

[NyxCharon]

Do you have a flash drive or other usb storage device plugged in to the computer?
Make sure your BIOS is set to have your HDD boot first.

Are you using A UEFI Bios or a standard one?

Which version of Ubuntu? Did you choose any non standard options during the install, for instance where grub(linux bootloader) is installed?

EDIT: I see it's a sometimes thing. That's probably from a USB device being plugged in on startup. Happens to me all the time when I forget to remove something since I have USB selected first in BIOS.