So a quick update (2 days later):
I swapped the 2 GeForce GTX1060 Turbo cards for a new Radeon AMD WX7100, and wow the change is amazing. Here are the initial findings (5 Pros and 1 possible con):
1) All graphics, windows animation including scrolling and max/minimizing are so much smoother
2) In Nuendo 10, everything is smooth now. No more jittering cursors, or pausing when I open an editor, and the Nuendo graphics are so much more detailed with finer resolution
3) Rendering in Premiere Pro is about 5 times faster, and again no more jittering video playback
4) The new Radeon WX7100 takes a single slot on my motherboard, freeing up 3 more slots. The GTX1060's require 2 slots each. So I can now do away with the external thunderbolt expansion box and bring my 2 UAD Octos internally, yay!!
5) The new WX7100 fan is quieter than the GTX1060s, despite various reviews complaining that the fan is fairly noisy (I guess it depends which GFX cards you're comparing to).
I've had one crash so far where all 4 screens went a sort of grey misty colour and the PC froze. I don't know if this is down to the Radeon card or the latest Windows update though.
For info: My PC-workstation is running Windows10, with dual CPU (2696v3), 128GB RAM, and 4 displays (x3 4k and x1 2k).
So, I'll keep the new Radeon card installed for a week and see how things go, but early signs are that the GeForce GTX1060's were not up to the job, despite their spec being overkill for anything I do on my PC.