Solid Sierra build suddenly won't boot - Graphics??

[wckdkarma]

I disconnected my HD and connected a brand new drive to create a "spare" installation. I couldn't get that installation to boot so I put it away for a later time and reconnected main HD, which has been working great for at least the past6 months. Now I cannot get it to boot consistently.
I have to cycle between boot arguments ( -v, nvda_drv=1, nv_disable=1 and combinations of these three) about a dozen times before it will boot.
My errors are usually "error loading kernel cache" or "couldn't allocate runtime area".
I've tried to run MultiBeast again but it fails every time.
I don't know what to try next.

Would really appreciate any guidance.
thanks in advance!

 

[wckdkarma]

Any ideas????
I have been able to boot into safe mode but I'm not sure what I should do at that point?

 

[LA3]

Any ideas????
I have been able to boot into safe mode but I'm not sure what I should do at that point?

Have you chosen the boot order in your Gigabyte mother board BIOS of the drive you want to boot first?

 

[wckdkarma]

Have you chosen the boot order in your Gigabyte mother board BIOS of the drive you want to boot first?

Yes.
I've removed my graphics card (GTX 970) and can get a normal boot.
I've reinstalled Nvidia drivers, re-inserted graphics card, reboot, select Nvidia graphics then I might get 1 normal boot but rarely two and most often, I have to remove the graphics card again to get a normal boot.
Weird thing is I have a second machine (my mom's) with exactly the same hardware showing the same symptoms, which is what prompted me to create a new clean installation of Sierra. I don't know what happened prior to her's refusing to boot, but the error messages appeared to be the same. Both of these machines have been running really well since upgrading to Sierra about 6 months ago.
This is my main machine and its been down now for almost a week, I'm really anxious to get it back up and running.
Thanks so much for your help!