What time is considered day and night

I can see why there are several posts suspecting it might be your video card overheating... the behavior you describe kind of suggests it... but it might be power settings.

Check the 'Manage 3D Settings' category in the NVIDIA Control Panel and make sure your Power Management Mode is set to Optimal Power. This is the default setting, so its kind of a long shot, but when it comes to troubleshooting, you need to be thorough.

With that 'thorough' concept in mind, test the stability during game play. If you can, find a large group of wolves or nekkers or something... try to cause graphic intensive activity. If it really is overheating, the GPU fan should cause a noticeable sound difference, not to mention the game would crash soon after. This serves the purpose of eliminating the cause more than it attempts to resolve the issue, but if you have overheating hardware, it needs to be addressed immediately.

Here's a final suggestion should your issue persist. Find the game installation folder... which, I think, by default is

C:\Program Files(x86)\The Witcher 3 Wild Hunt

and navigate to the bin\config\base folder. Now locate the rendering.ini file and create a copy somewhere... anywhere.

Once you have the file backed up, Right-Click and Edit it. Either use the search function or skim the variables (under the [Rendering] block) until you spot TextureMemoryBudget=500 and increase this to 1024. Save the file, and test out the game again.

Hope this helps. If not, I think the error information needs to be analyzed a little more closely... in an attempt to narrow down and pinpoint a more precise cause.

You can instantly begin, as long as you are done with the Prologue. The difficulty will either scale with your level 30 character, or your level of difficulty in NG+.

In your case, after you are done with the Prologue, you'll start the game with a quest in your journal and the enemies will be associated with your difficulty level at the time the journal entry was created.