Thought I had solved it before, but still get lag every minute or so,
for about half a second it feels like the game is 1fps but fps counter still states over 100.

GPU useage doesn;t change, nort does CPU during these lag spikes, it's doing my head in and I cannot figure out whats causing it.

I have a 1080ti
Ryzen 1700
16GB ram

Every other game works perfectly, Shadow of tomb raider and latest f1 game run perfectly, So not my computer, normally get between 100 and 140fps in witcher 3 with everything set to max, tried turning settings down and off, no change. Any ideas?

Lso tried locking framrate to 60... still the same.

i would try several different gpu drivers. i am running this game on a 960 without the problem you mentioned.

set the FPS limiter to unlimited, and set all of the graphics filters to "High". I have 2 970's in sli, and this happened when "foilage visibility" was set to ultimate. Changed all settings to high and have a super smooth experience. Hope this helps

P.S. Nvidia hairworks is WORTHLESS and actually makes hair look terrible. set it to zero, and no AA for hairworks. Its garbage.