On Mon, 17 Sep 2018 10:22:15 -0700, mike wrote:

Dual boot is a minefield because both operating systems think they want
to control the boot process.

I wrote a couple of tutorials which may help the OP if there's an
intermittend hardware issue of some sort.

For example, this was an issue with RAID bits left on the hard drive:
Why doesn't Ubuntu 18.04 ask to install next to Windows 10 Pro
single HDD as a dual boot?
https://groups.google.com/forum/#!topic/alt.os.linux/D7E7FQ1NLNk%5B1-25%5D

It sounds like the OP worked around the problem where it's well known that
the dual-boot process (I'll call it "GRUB" for lack of a better term) does
strange things when it "thinks" there is a hardware problem.
Have you ever seen a Grub that wouldn't respond to the keyboard?
https://groups.google.com/d/msg/alt.os.linux/yQmtemiHcVk/glo2kUA2CAAJ

This thread shows what happens when "GRUB" thinks that a RAID controller
was in existence (when it wasn't):
Quick question asked of how to install ubuntu (as a dual boot)
https://groups.google.com/d/msg/alt.os.linux/5Xl7DPopNnE/fBpSed9gAwAJ

This thread shows what happens when "GBUB" thinks that the keyboard has an
issue (when it doesn't):
Proactive setup instructions for dual boot with Windows & Ubuntu
via Grub using legacy keyboards
https://groups.google.com/d/msg/alt.os.linux/WltumTFjzU8/98HTzaNuCAAJ

In the first case, the solution was to literaly wipe every single bit on
the hard drive, including the normally inaccessible bits at the beginning
and end.

In the latter case, the solution was to mess with the BIOS.

Hope this helps the OP. I realize the solution I wrote up are detailed, and
specific to two hardware issues:
a. Grub wouldn't recognize the USB keyboard (under the given circumstances)
b. Grub wouldn't recognize the HDD (under the given circumstances)

I only present these to the OP in terms of how to approach debugging &
resolution, since both problems were successfully debugged & resolved.