I have about the same setup and I'm getting way worse frames than you, on ultra 1440p settings 60-120fps, it stays somewhat stable for a second(about 100fps) and then stutters to 60-70 with massive lag and it keeps happening again and again..
Really frustrating.
But one thing I wonder is that if I drop resolution to 1920x1080, my frames get even worse! 50-90fps..
1080ti Gaming X (no oc)
i7 8700k (no oc)
16gb ram
SSD
144hz ips G-sync panel
Nvidia panel G-sync on, V-sync on and game V-sync off
No matter if I use DX11 or DX 12, same results
Temps under control
Tried fixing the game on Origin, no results
This isnt normal, am I doing something wrong or? Can anyone help?