To start, I'd like to say this post may be completely stupid, useless or some other negative word, but it's an idea, and how I traced my somewhat similar problem almost certainly to my PSU.
Would a Windows reinstall be a massive pain for you? That is honestly my favourite way of dealing with potentially bad drivers.
As I'm not really sure how going from 1 Nvidia card to another much newer one is handled with regards to drivers it's my first thought.