Changing and installing grub config
Edit /etc/default/grub. Modify file in the directory /etc/grub.d. Afterwards run update-grub (which just runs grub-mkconfig -o /boot/grub/grub.cfg). Do not edit /boot/grub/grub.cfg. This file is generated by grub-mkconfig.
In /etc/default/grub I like to set the following values:
Hold the shift key down while the system is powering up. This will bring up the GNU GRUB boot menu.
Try the boot precessing while holding shift a few more times. It may not always work. If that still doesn't work then try pressing the Esc key instead. Apparently in certain situations GRUB2 can tell that it cannot determine if the shift key is pressed or not, so it waits an additional 3 seconds and waits to see if the Esc key is pressed. When this happens you may see a cursor blinking on the screen while GRUB2 waits for 3 seconds.