2017.4.30f1 lighting is glitchy

I have been working on a flight game for the past couple years and we have been building the project in 5.4.6f3 but have experienced stability issues, so we decided to upgrade the project to Unity version 2017.4.30f1. Without changing anything after porting to the new version, while we fly around we see many glitches with the lighting. As you move around the scene, some objects flicker as if some lights are randomly not being rendered in some frames. Sometimes shadows disappear for no reason (not distance based) and the illumination of objects will change (bright, then dark, then bright again). This is really halting our production as we cant go backwards due to application crashes (2017.4.30f1 doesnt crash but 5.4.6f3 does) and the new version is visually unacceptable. We have tried everything from increasing the number of max lights, to reducing the lights to a minimum number possible (2), and even using lights that light certain objects exclusively. No matter how we set it up, we still see these lights glitching out, causing the scene color to flicker, shadows disappearing, and specular highlights flickering. We would go to an even newer version of Unity but we cant because we are using the last version which supports our networking scheme which is necessary for the game. We really feel like we're hitting a brick wall here... Right now I am attempting to bake the lights into our scene but this is a road we didn't really want to go down, for various reasons. Our scene is very big (4km x 4km) with Unity terrain and lots of clutter objects, so computation is taking ~24 hours per bake. Any ideas? Has anyone else experienced lighting issues with 2017.4.30f1?

 

OK so I have turned off all lights in the scene and when I hit play, the lights still flicker! What the hell is going on here?? I have all lights turned off and they just randomly flicker on and off. This is nuts... :<
EDIT: The glitching is visible in the editor Scene View as well as the Game View and Camera.

EDIT 2: The glitching also happens when the editor is not even in Play Mode.

 

Usually when I see flickering, its because I've hit the max on pixel lights and its reverting back to vertex lighting.

Can you make a video to show the issue? That would help out a lot.

Also, are you sure that the lights are flickering? I ask because if they are off, how are they turning back on?